docs: Correct commit summary length guidlines
docs: Correct commit summary length guidlines

pw_presumit has been enforcing a 72-character column limit for commit
bodies AND summaries.

Change-Id: I252b18104f749f687bf684d9eb5d4b809b70376f
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/97683
Pigweed-Auto-Submit: Armando Montanez <97ef6fdb231c129ac8827d9f0c4106bfbf57f74f@google.com>
Commit-Queue: Auto-Submit <14637eda5603879df170705285bf30f5996692f0@pigweed.google.com.iam.gserviceaccount.com>
Reviewed-by: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed