fix typo: method receiver should contain type arguments, not parameters

From what I understand, type parameters can only be used in a type or a function declaration. Thus, in a method receiver, one can only find type arguments. Having `*Iterator[K, V any]` there looks like a little typo to me. The go2go Playground seems to agree with me: [before](https://go2goplay.golang.org/p/dekNQ6Xy8XM) there's a parse error while there's no [after](https://go2goplay.golang.org/p/4B8iTq2AeET).

Change-Id: I53e36fca75d3573f56bdb89fbc48bf4284502ffd
GitHub-Last-Rev: 4f80c8e8b0c06a9c5457c1cdbe6c2cbfbb515a84
GitHub-Pull-Request: golang/proposal#27
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/253819
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/design/go2draft-type-parameters.md b/design/go2draft-type-parameters.md
index df38026..1a1f9c5 100644
--- a/design/go2draft-type-parameters.md
+++ b/design/go2draft-type-parameters.md
@@ -3274,7 +3274,7 @@
 // Next returns the next key and value pair. The bool result reports
 // whether the values are valid. If the values are not valid, we have
 // reached the end.
-func (it *Iterator[K, V any]) Next() (K, V, bool) {
+func (it *Iterator[K, V]) Next() (K, V, bool) {
 	kv, ok := it.r.Next()
 	return kv.k, kv.v, ok
 }