Fix typo in tutorial-1

[liu-mengyang]

Motivation

Typo in tutorial-1.

Modification

A example -> An example.

Checklist

• Pre-commit or other linting tools are used to fix the potential lint issues.
• The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMDet3D.
• The documentation has been modified accordingly, like docstring or example tutorials.

[CLAassistant]

All committers have signed the CLA.