fix message when monster's two-handed weapon welds

K3610 reported to devteam:
When you see a monster wield a cursed two-handed weapon,
the weapon "welds itself to the foo's hand" instead of its "hands."

Observed on hill orcs wielding a cursed two-handed sword.

src/weapon.c

@@ -836,9 +836,13 @@ mon_wield_item(struct monst *mon)
             newly_welded = mwelded(obj);
             obj->owornmask &= ~W_WEP;
             if (newly_welded) {
+                const char *mon_hand = mbodypart(mon, HAND);
+
+                if (bimanual(obj))
+                    mon_hand = makeplural(mon_hand);
                 pline("%s %s to %s %s!", Tobjnam(obj, "weld"),
                       is_plural(obj) ? "themselves" : "itself",
-                      s_suffix(mon_nam(mon)), mbodypart(mon, HAND));
+                      s_suffix(mon_nam(mon)), mon_hand);
                 obj->bknown = 1;
             }
         }