bsd.port.mk: fix comment typo and excess full-stop in IGNORE (FLAVORS)

This is to fix a cacche type to ccache in a comment, and the double
full-stop on the console when trying to build an unsupported FLAVOR:

> py27-example-9.8.7 Unknown flavor 'py27', possible flavors: py37..

Approved by: portmgr@ (bapt@, on reviews.f.o)
Differential Revision: https://reviews.freebsd.org/D24588