spec: clarify “Constant expressions” for untyped operands

This change addresses the grammatical complexity described in
https://groups.google.com/forum/#!topic/golang-dev/RmP-LMC3g58.

Change-Id: Ib292b4ca9c880c7c1c8c992e7c033a0f8f951f2c
Reviewed-on: https://go-review.googlesource.com/106855


Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>