git-gui: Correct stock message for 'Invalid font specified in %s'

git-gui: Correct stock message for 'Invalid font specified in %s'

This particular message is talking about a specific option in the
configuration file named "gui.$name".  This option is not localized
so we cannot localize the "gui." that denotes the section the option
$name is found within.  Currently there are no plans to localize the
configuration options for git-gui, but if that were to change in the
future then it would be necessary to localize not only the "gui."
section prefix but also the $name (fontui and fontdiff).

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>