CodeReviewComments: recommend rand.Text for keys

This was added in https://go.dev/issue/67057 expressly
for the purpose of generating keys.

Change-Id: I5c63e7086649801421bc977e50bf619ed9078439
Reviewed-on: https://go-review.googlesource.com/c/wiki/+/655636
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Commit-Queue: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>