Replace references to "master" branch with "main"

I think the move to calling it "main" in the CrOS tree is far enough
along that this can be done safely.

BUG=b:162024466
TEST=none

Change-Id: Ied90329c19b70303db90b81665d1229b2b08184d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/mttools/+/2664793
Tested-by: Harry Cutts <[email protected]>
Auto-Submit: Harry Cutts <[email protected]>
Reviewed-by: Sean O'Brien <[email protected]>
Commit-Queue: Harry Cutts <[email protected]>
2 files changed