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
Warning removed for missmatched indentation
Fix documentation of extract_handler_and_format_and_variant [ci skip]
Merge pull request #14875 from prathamesh-sonpatki/fix-extract-handler-format-variant-doc

Fix documentation of extract_handler_and_format_and_variant [ci skip]
Merge pull request #14874 from arunagw/aa-warning-removed-missmatched

Warning removed for missmatched indentation