update_gpu_drivers: Fix main branch name

We don't use m/master anymore.

BUG=b:212557635
TEST=./update_gpu_drivers.sh --package mali-drivers-bifrost

Change-Id: I5c8b451657667ac33cca9e2f0ae898a9d8e2d1ec
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/3358234
Tested-by: Fei Shao <fshao@chromium.org>
Auto-Submit: Fei Shao <fshao@chromium.org>
Reviewed-by: Hsin-Yi Wang <hsinyi@chromium.org>
Commit-Queue: Hsin-Yi Wang <hsinyi@chromium.org>
1 file changed