Update main.c (#618)

Grammar, like #607.

vector/v.category/main.c

@@ -76,7 +76,7 @@ int main(int argc, char *argv[])
     G_add_keyword(_("category"));
     G_add_keyword(_("layer"));
     module->description =
-	_("Attaches, deletes or reports vector categories to map geometry.");
+	_("Attaches, deletes or reports vector categories to/from/of map geometry.");
 
     in_opt = G_define_standard_option(G_OPT_V_INPUT);