form_helper documentation using article instead of post

Documentation previously uses post which is confusing as the
methods/verb is also post.

Before
<form action="/posts" method="post">

After
<form action="/articles" method="post">

Method
Worked through a new Rails app with scaffold of article
and checking the output of the HTML. Other HTML changes
make it more authentic:
/> => at the end of the input

Author: notapatch