bug#15818: src/dfa.c: *MALLOC comment block: Clarify comment wording

[Jim Meyering]

Thanks for the reminder.
Here's the commit I've created for you, along with a similarly trivial
one I spotted recently.
Please confirm that the USER NAME and <email address> I used in your
commit are ok.
In any future patches, please submit the output of "git format-patch
...", including a log message, per guidelines in HACKING.