Replaced <br> with <br /> for consistency.

Most files used <br />, getting_started changed in mid-document, so I
went with the majority usage.
Added note about singular/plural distinction.

To a Rails noob, this was really confusing---where was "post" coming
from?---so I added a short note to note it (and move on).  Also
fixed a grammatical error and corrected the output of `rake routes`.