Add missing comma in example in lua_api.txt (#12339)

doc/lua_api.txt

@@ -2012,7 +2012,7 @@ Example definition of the capabilities of an item
         max_drop_level=1,
         groupcaps={
             crumbly={maxlevel=2, uses=20, times={[1]=1.60, [2]=1.20, [3]=0.80}}
-        }
+        },
         damage_groups = {fleshy=2},
     }