Update InputUnion RFC to OneOf RFC

The GraphQL RFC has changed from InputUnions:

- https://github.com/graphql/graphql-spec/issues/488

to OneOf Input Objects:

- https://github.com/graphql/graphql-spec/pull/825

doc/development/api_graphql_styleguide.md

@@ -1801,8 +1801,8 @@ def ready?(**args)
 end
 ```
 
-In the future this may be able to be done using `InputUnions` if
-[this RFC](https://github.com/graphql/graphql-spec/blob/master/rfcs/InputUnion.md)
+In the future this may be able to be done using `OneOf Input Objects` if
+[this RFC](https://github.com/graphql/graphql-spec/pull/825)
 is merged.
 
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