fix typo: method receiver should contain type arguments, not parameters
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 <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/proposal,golang/proposal