Fix typo in _expand_fspec_to_fun

Introduced by fefce4a0, typo invalidated catfun and ffdoc

src/ffoo.sh.skel

@@ -90,7 +90,7 @@ ffoo() {
             ;;
         _expand_fspec_to_fun)
             local mod
-            mod==$(ffoo _expand_fspec_to_mod "$2")
+            mod=$(ffoo _expand_fspec_to_mod "$2")
             echo "${2##$mod.}"
             ;;
         _expand_path_to_fspeclist)