The method must override or implement a supertype method

It means that you don't need the override annotation since you aren't overriding or implementing something to that method. Therefore, you should merely delete

@Override

To override a method the signature needs to match that of the super class. Replace

public String getArmorTexture(ItemStack stack, Entity entity, int slot, int type) {

with

public String getArmorTexture(ItemStack stack, Entity entity, int slot, String type) {

If you are using Eclipse, try closing and opening it again. The error goes away.