cmd/stringer: tweak "Code generated by" comment to match new standard

See https://golang.org/issue/13560 for the full discussion.

The actual change is just the addition of a final period.

Update golang/go#13560

Change-Id: Icc2f52b67181de344aa5107f94faa0e739ff993c
Reviewed-on: https://go-review.googlesource.com/38415
Reviewed-by: Ian Lance Taylor <iant@golang.org>
1 file changed