fix styling (ATX) in README for consistency (#3589)

reference: https://github.com/DavidAnson/markdownlint

README.md

@@ -1,4 +1,5 @@
 # OpenRefine
+
 [![DOI](https://zenodo.org/badge/6220644.svg)](https://zenodo.org/badge/latestdoi/6220644)
 [![Join the chat at https://gitter.im/OpenRefine/OpenRefine](https://badges.gitter.im/OpenRefine/OpenRefine.svg)](https://gitter.im/OpenRefine/OpenRefine) ![Java CI](https://github.com/OpenRefine/OpenRefine/workflows/Java%20CI/badge.svg) [![Coverage Status](https://coveralls.io/repos/github/OpenRefine/OpenRefine/badge.svg?branch=master)](https://coveralls.io/github/OpenRefine/OpenRefine?branch=master) [![Translation progress](https://hosted.weblate.org/widgets/openrefine/-/svg-badge.svg)](https://hosted.weblate.org/engage/openrefine/?utm_source=widget) [![Total alerts](https://img.shields.io/lgtm/alerts/g/OpenRefine/OpenRefine.svg?logo=lgtm&logoWidth=18)](https://lgtm.com/projects/g/OpenRefine/OpenRefine/alerts/)
 
@@ -8,49 +9,50 @@ the web. All from a web browser and the comfort and privacy of your own computer
 
 [<img src="https://github.com/OpenRefine/OpenRefine/blob/master/graphics/icon/open-refine-320px.png" align="right">](http://openrefine.org)
 
-Download
------------------------
+## Download
+
 * [OpenRefine Releases](https://github.com/OpenRefine/OpenRefine/releases)
 
-Run from source
-------------------
+## Run from source
+
 If you have cloned this repository to your computer, you can run OpenRefine with:
+
 * `./refine` on Mac OS and Linux
 * `refine.bat` on Windows
 
 This requires JDK 8 and Apache Maven.
 
-Documentation and Videos
--------------------------
+## Documentation and Videos
+
 * [User Manual](https://docs.openrefine.org)
 * [FAQ](https://github.com/OpenRefine/OpenRefine/wiki/FAQ)
 * [Official Website and tutorial videos](http://openrefine.org)
 
-Contributing to the project
----------------------------
+## Contributing to the project
+
 * [Developers Guide & Architecture](https://github.com/OpenRefine/OpenRefine/wiki/Documentation-For-Developers)
 * [Contributing Guide](https://github.com/OpenRefine/OpenRefine/blob/master/CONTRIBUTING.md)
 * [Project Governance](https://github.com/OpenRefine/OpenRefine/blob/master/GOVERNANCE.md)
 
-Contact us
-----------
+## Contact us
+
 * [Mailing List](https://groups.google.com/forum/#!forum/openrefine)
 * [Twitter](http://www.twitter.com/openrefine)
 * [Gitter](https://gitter.im/OpenRefine/OpenRefine)
 * [Matrix (bridged from Gitter)](https://matrix.to/#/#OpenRefine_OpenRefine:gitter.im)
 
-Licensing and legal issues
---------------------------
+## Licensing and legal issues
+
 OpenRefine is open source software and is licensed under the BSD license
 located in the [LICENSE.txt](LICENSE.txt). See the folder `licenses` for information on open source
 libraries that OpenRefine depends on.
 
-Credits
--------
+## Credits
+
 This software was created by Metaweb Technologies, Inc. and originally written
 and conceived by David Huynh <dfhuynh@google.com>. Metaweb Technologies, Inc.
 was acquired by Google, Inc. in July 2010 and the product was renamed Google Refine.
-In October 2012, it was renamed OpenRefine as it transitioned to a 
+In October 2012, it was renamed OpenRefine as it transitioned to a
 community-supported product.
 
 See [AUTHORS.md](./AUTHORS.md) for the list of OpenRefine contributors and [CONTRIBUTING.md](./CONTRIBUTING.md) for instructions on how to contribute yourself.