CONTRIBUTING.md: Clarify clarification on periods

Commit 9428c28f added a note in
`CONTRIBUTING.md` saying that, "For consistency, there should not be a
period at the end of the commit message."

I believe it would be more correct (with respect to consistency) to
say that there should not be a full stop/period at the end of the
commit message's *summary line* (the first line of the commit message)
(indeed, the example commit message just above this note clearly does
use full stops outside of its summary line), and this patch changes
`CONTRIBUTING.md` accordingly.