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'].
Merge pull request #85 from FeFiFoFu/patch-2

Update ajax.rst