Typo in print format.

This causes a segfault when addwatch fails due to a disappeared
directory. Eg: often caused by a race condition with a short lived
directory created by Firefox' safebrowsing subsystem:
~/.cache/mozilla/firefox/pr0f1l3.default/safebrowsing-to_delete/
Merge pull request #255 from plouj/patch-1

Typo in print format.