Update external.mdx

No longer need the (. to uncurry in the tips and tricks section example.

Author: Darkle

pages/docs/manual/latest/external.mdx

@@ -50,7 +50,7 @@ Once declared, you can use an `external` as a normal value, just like a let bind
 @val external document: 'a = "document"
 
 // call a method
-document["addEventListener"](."mouseup", _event => {
+document["addEventListener"]("mouseup", _event => {
   Console.log("clicked!")
 })