Fix keyword format.

Author: Kevin Deldycke

docs/yaml-config.rst

@@ -151,7 +151,7 @@ python.extra_requirements
 * Type: List
 
 List of `extra requirements`_ sections to install, additionnaly to the
-`package default dependencies`_. Only works if `python.pip_install` option
+`package default dependencies`_. Only works if ``python.pip_install`` option
 above is set to `True`.
 
 Let's say your Python package has a ``setup.py`` which looks like this: