Slightly rephrased sentence to emphasize the contents of the
Unicode categories w/o repeating the full category name each
time.

Fixes #13414.

Change-Id: Icd32ff1547fa81e866c5937a631c3344bb6087c6
Reviewed-on: https://go-review.googlesource.com/18265Reviewed-by: Rob Pike <r@golang.org>