Rename the docs section for Rectangles.

Rename the docs section for Rectangles.

The web page currently assumes it's called 'rect' rather than
'rectangles', because the web-page building script uses the first
field of each line of gamedesc.txt, same as the Unix binary name.
Rather than add another confusingly-almost-identical field to that
file, it's easier to just rename this one docs section to make the
assumption of equality hold.