all: use the indefinite article an in comments

[callthingsoff]

This is a follow up of CL 530120.

[gopherbot]

This PR (HEAD: b4154fa) has been imported to Gerrit for code review.

Please visit Gerrit at https://go-review.googlesource.com/c/go/+/531136.

Important tips:

• Don't comment on this PR. All discussion takes place in Gerrit.
• You need a Gmail or other Google account to register for Gerrit.
• To change your code in response to feedback:
  • Push a new commit to the branch used by your GitHub PR.
  • A new "patch set" will then appear in Gerrit.
  • Respond to each comment by marking as Done in Gerrit if implemented as suggested. You can alternatively write a reply.
  • Critical: you must click the blue Reply button near the top to publish your Gerrit responses.
  • Multiple commits in the PR will be squashed by GerritBot.
• The title and description of the GitHub PR are used to construct the final commit message.
  • Edit these as needed via the GitHub web interface (not via Gerrit or git).
  • You should word wrap the PR description at ~76 characters unless you need longer lines (e.g., for tables or URLs).
• See the Sending a change via GitHub and Reviews sections of the Contribution Guide as well as the FAQ for details.

[gopherbot]

Message from Bryan Mills:

Patch Set 1: Auto-Submit+1 Code-Review+2 Commit-Queue+1

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/531136.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Go LUCI:

Patch Set 1:

Dry run: CV is trying the patch.

Bot data: {"action":"start","triggered_at":"2023-09-26T19:05:23Z","revision":"79c3572da547e4f6ca86393c8985ccc93f0c398a"}

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/531136.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Bryan Mills:

Patch Set 1: -Commit-Queue

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/531136.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Go LUCI:

Patch Set 1:

This CL has passed the run

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/531136.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Go LUCI:

Patch Set 1: LUCI-TryBot-Result+1

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/531136.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

This PR is being closed because golang.org/cl/531136 has been merged.