tools: clarify commit message linting

Clarify in Travis results that the commit message linting is for the
commit message and not something else.

PR-URL: https://github.com/nodejs/node/pull/23742


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Vladimir de Turckheim <vlad2t@hotmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>