doc/go1.16: add missed heading tag in vet section
doc/go1.16: add missed heading tag in vet section

Add missed heading tag in CL 276373.

For #40700

Change-Id: Ida9e8861589bbc296a5a1cecbf9fe33fa09ed0ca
GitHub-Last-Rev: d218f8d4b70b20c30422863db7bed3683e3218e6
GitHub-Pull-Request: golang/go#44111
Reviewed-on: https://go-review.googlesource.com/c/go/+/289869
Reviewed-by: Tim King <ee8a1d1efb05d8171fc0140ced15d455e83941c5@google.com>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Trust: Tim King <ee8a1d1efb05d8171fc0140ced15d455e83941c5@google.com>
Trust: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/go,nawawi/go,nawawi/go,cloudflare/go,nawawi/go,cloudflare/go,cloudflare/go,golang/go,golang/go,golang/go,nawawi/go,cloudflare/go,cloudflare/go,golang/go,CAFxX/go,golang/go,CAFxX/go,CAFxX/go,CAFxX/go,nawawi/go,nawawi/go,CAFxX/go,cloudflare/go,CAFxX/go,CAFxX/go,nawawi/go,nawawi/go,CAFxX/go,golang/go,cloudflare/go,cloudflare/go,golang/go