Fix markup for pre-commit install block

Author: living180

docs/contributing.rst

@@ -103,7 +103,7 @@ Style
 The Django Debug Toolbar uses `black <https://github.com/psf/black>`__ to
 format code and additionally uses flake8 and isort. The toolbar uses
 `pre-commit <https://pre-commit.com>`__ to automatically apply our style
-guidelines when a commit is made. Set up pre-commit before committing with:
+guidelines when a commit is made. Set up pre-commit before committing with::
 
     $ pre-commit install