rfc1: suggest "main" not "master" branch

Problem: RFC 1 refers to the "master" branch, but the preferred
name for that is now "main".

Reword so that "main" is a SHOULD requirement.

Author: garlick

spec_1.rst

@@ -182,7 +182,7 @@ Release Process
 Creating Stable Releases
 ========================
 
--  The project SHALL have one branch ("master") that always holds the latest in-progress version and SHOULD always build.
+-  The project SHALL have one branch (that SHOULD be named "main") that always holds the latest in-progress version and SHOULD always build.
 
 -  The project SHALL NOT use topic branches for any reason. Personal forks MAY use topic branches.