spec: for non-constant map keys, add reference to evaluation order section
The section on map literals mentions constant map keys but doesn't say
what happens for equal non-constant map keys - that is covered in the
section on evaluation order. Added respective link for clarity.
Fixes #19689.
Change-Id: If9a5368ba02e8250d4bb0a1d60d0de26a1f37bbb
Reviewed-on: https://go-review.googlesource.com/38598 Reviewed-by: Rob Pike <r@golang.org> Reviewed-by: Matthew Dempsky <mdempsky@google.com> Reviewed-by: Ian Lance Taylor <iant@golang.org>