gapplication-tool: fix a typo

This must have never worked.

https://bugzilla.gnome.org/show_bug.cgi?id=727928
gapplication-tool: ensure object paths are valid

Hyphens are not valid characters for DBus object paths; see the similar
code in gapplicationimpl-dbus.c

https://bugzilla.gnome.org/show_bug.cgi?id=727928