exercise, fix typos

[getting started] dialog box is not automatic, is provided through jquery_ujs, which is necessary to have in application layout

[ci skip]

<plus> is nonstandard Textile; RailsGuides::TextileExtensions::plusplus
is the method that causes multiple code-formatted `+` symbols to
generate crap like

    <code>1 <notextile><tt>+</tt></notextile> 2 ...</code>

instead of the expected output:

    <code>1 + 2 ...</code>

[getting started] some more handholding around where exactly the error would appear when a post is invalid

[getting started] re-add new action back to PostsController, explain that it'll be explained in a short while