Fix an old bug where the config is read incorrectly

This should be "DamageModifier" not "DamagerModifier.". As seen here https://github.com/mcMMO-Dev/mcMMO/blob/dbc7b98b0d2d7793c20b6dd1e5ee2ac7aadc8fd2/src/main/resources/advanced.yml#L140 . Thanks to @matagin for pointing this out.