querying.rst: update ManyToManyField module belong
Is fields the right ref?
 No sir! It's `extra-fields`! fixing that up, sorry!
 push-forced the changes, but i've not yet built the docs locally so this is untested
 installed sphinx, `make html`, links are working! should I check anything else?
 Thanks a lot!

ManyToManyField is now on the playhouse.fields module, docs were slightly misleading
