Minor update to auto-roller infra docs. Bug: angleproject:1944 Change-Id: I06e613f002d390e85faadc9bec903e83fdd9fdec Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1560596 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>