Fix test instructions formatting.

Author: melissawm

docs/contribute/index.rst

@@ -58,10 +58,9 @@ Pull request guidelines
    `conftest.py <https://github.com/melissawm/sphinx-tags/tree/main/test/conftest.py>`__
    for details on testing setup.
 
-   To run tests, use
-   ```
-   pytest
-   ```
+   To run tests, use::
+
+      pytest
 
 5. **Commit your changes and send your pull request as usual.**