docs: update README.md (#174)

The log level for warning is WARN, not WARNING
If you use WARNING it is ignored.

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:
- [ ] Make sure to open an issue as a [bug/issue](https://github.com/googleapis/java-logging-logback/issues/new/choose) before writing your code!  That way we can discuss the change, evaluate designs, and agree on the general idea
- [ ] Ensure the tests and linter pass
- [ ] Code coverage does not decrease (if any source code was changed)
- [ ] Appropriate docs were updated (if necessary)

Fixes #<issue_number_goes_here> ☕️

README.md

@@ -79,7 +79,7 @@ See [Logback filters](https://logback.qos.ch/manual/filters.html#thresholdFilter
     <log>application.log</log>
 
     <!-- Optional: defaults to "ERROR" -->
-    <flushLevel>WARNING</flushLevel>
+    <flushLevel>WARN</flushLevel>
 
     <!-- Optional: auto detects on App Engine Flex, Standard, GCE and GKE, defaults to "global". See supported resource types  -->
     <resourceType></resourceType>