Update ajax.rst

Template html has \<b> tags, but in the text discussion, you refer to them as \<strong>.  The real source code uses \<b>, but I think \<strong> is a best practice. 

The python code example is not up-to-date with the real source code.  Specifically, line 379 of views.py: request.GET['cat_id'] versus request.GET['category_id'].