Fix code & grammar in Form Helpers Guide

in the "Forms to External Resources" section [skip ci]
Merge pull request #14873 from jonatack/patch-8

Fix code & improve grammar in "Forms to External Resources" [ci skip]
Fix custom join_table name on habtm reflections

When used a custom join_table name on a habtm, rails was not saving it
on Reflections. This causes a problem when rails loads fixtures, because
it uses the reflections to set database with fixtures.
Merge pull request #14853 from kassio/kb-fix-custom-join-table-habtm

Fix to use custom join table in habtm
Fix how to compute class name on habtm namespaced.

Thank's for @laurocaetano for the help with tests. :smiley:

Fixes #14709