Fix some master -> main references in docs

Bug: skia:12478
Change-Id: I4e2bc1eb441c19d7b4cf2bcea65b852f7f0aa59b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/453136
Reviewed-by: Ravi Mistry <rmistry@google.com>
Commit-Queue: Eric Boren <borenet@google.com>
21 files changed