Grammar fix (#550)

Add a missing comma

Author: stephanie-eng

doc/how_to_contribute/how_to_contribute.rst

@@ -2,7 +2,7 @@ Contributing
 ============
 
 These how-to guides outline steps for contributing to MoveIt and this website.
-If you are looking for How-To Guides for using MoveIt see :doc:`/doc/how_to_guides/how_to_guides`.
+If you are looking for How-To Guides for using MoveIt, see :doc:`/doc/how_to_guides/how_to_guides`.
 
 .. toctree::
    :maxdepth: 1