CommitMessage: synchronize line length recommendation This brings it in line with gerrit and /doc/contribute, updated in CL 727860. Change-Id: Icf75750baa92d0564253af8511a745c06a6a6964 Reviewed-on: https://go-review.googlesource.com/c/wiki/+/729840 Reviewed-by: Sean Liao <sean@liao.dev>