Prevent unwanted c-format in go-pattern-selector.c

Closes: #75 (closed)

See solution:

1. Run intltool-update -p inside po/ dir — without the changes from this merge requests
2. Check po/goffice.pot for '75% background' and notice the , c-format flag above this and nearby strings
3. Now apply the changes from this merge request
4. Redo steps 1 and 2, to find a , no-c-format flag where there was c-format flag.

From GNU Gettext docs:

xgettext of course could make a wrong decision the other way round, i.e. a string marked as a format string actually is not a format string. In this case the msgfmt might give too many warnings and would prevent translating the .po file. The method to prevent this wrong decision is similar to the one used above, only the comment to use must contain the string xgettext:no-c-format.