doc/Doxyfile: refactor "master" to "main"

Arm is committed to making the language we use inclusive, meaningful, and
respectful. This commit is part of the inclusive language refactoring series
where we replace offensive terminology with more inclusive ones.

Signed-off-by: Brett Warren <brett.warren@arm.com>
Change-Id: I4f274d5acbccd011bf0dcf82d7ace126b291fca0