delete double space from comment

Change-Id: I71af5c1149941575016f79a91269f128b1fc16af
GitHub-Last-Rev: aba8874bd362d05d6c29c8647049369dfcd796f5
GitHub-Pull-Request: golang/go#23851
Reviewed-on: https://go-review.googlesource.com/94415Reviewed-by: Ian Lance Taylor <iant@golang.org>