spec: every type has a method set (minor clarification)

The spec states that a type "may" have a method set associated with it.
Yet every type has a method set, which may be empty. This is clarified
later in the same paragraph. Be clear in the first sentence as well.

Per the suggestion from https://github.com/DQNEO.

Fixes #44318.

Change-Id: I6097b1c7062853e404b7fead56d18a7f9c576fc3
Reviewed-on: https://go-review.googlesource.com/c/go/+/292853


Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>