doc: slightly relax 50 character rule

Allow commit message first line to exceed 50 chars if necessary

PR-URL: nodejs/node#16523
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>

Author: jasnell

CONTRIBUTING.md

@@ -85,8 +85,8 @@ $ git commit
 The commit message should describe what changed and why.
 
 1. The first line should:
-   - contain a short description of the change
-   - be 50 characters or less
+   - contain a short description of the change (preferably 50 characters or less,
+     and no more than 72 characters)
    - be entirely in lowercase with the exception of proper nouns, acronyms, and
    the words that refer to code, like function/variable names
    - be prefixed with the name of the changed subsystem and start with an