docs: missing 'repeatable' in @goExtraField directive (#3150)

Author: alexandear

docs/content/recipes/extra_fields.md

@@ -58,7 +58,7 @@ directive @goExtraField(
 	type: String!
 	overrideTags: String
 	description: String
-) on OBJECT | INPUT_OBJECT
+) repeatable on OBJECT | INPUT_OBJECT
 ```
 
 Now you can use these directive when defining types in your schema: