spec: fix prose about promoted methods of embedded fields

spec: fix prose about promoted methods of embedded fields

The types of embedded fields must be named, but they don't
need to be defined types (e.g. if the type name is an alias).

Fixes #41687.

Change-Id: Ib9de65dfab0e23c27d8303875fa45c217aa03331
Reviewed-on: https://go-review.googlesource.com/c/go/+/406054
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>