cmd/api: make check pickier about api/*.txt

cmd/api: make check pickier about api/*.txt

We don't have a formatter for these files, so check here that
they are in the right form to allow 'cat next/*.txt >go1.X.txt'
at the end of each cycle.

Fix the api files that the check finds.

Change-Id: I0c5e4ab11751c7d0afce32503131d487313f41c0
Reviewed-on: https://go-review.googlesource.com/c/go/+/431335
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>