typo: external types s/function/type (#578)

Author: cobac

concepts/external-types/about.md

@@ -2,7 +2,7 @@
 
 External types can be used to refer to data types defined in other languages, such as Erlang or JavaScript.
 
-To define an external function declare a type but do not provide any constructors. This can then be used in the same way as any other type.
+To define an external type, declare a type but do not provide any constructors. This can then be used in the same way as any other type.
 
 ```gleam
 pub type OrderedDictionary(element)