use item.setAmount for .addAmount

Paper used to provide item.add(...), because of the switch to spigot, there is none
does middle finger emoji in github exist? I wanna show it to spigot

Author: coderFlameyosFlow

src/main/java/me/flame/menus/items/ItemEditor.java

@@ -165,7 +165,7 @@ public ItemEditor setAmount(int amount) {
      * @return the builder for chaining
      */
     public ItemEditor addAmount(int amount) {
-        this.item.add(amount);
+        this.item.setAmount(item.getAmount() + amount);
         return this;
     }