[COIL] Change code search type links from master to HEAD for chromiumos-assets

See this doc: http://go/chromiumos-coil-change for more info

Bug: 1214702
Change-Id: Icf4826fe24c6ae19953de72b4b2cdf7e16f3fe3c
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/chromiumos-assets/+/2928049
Auto-Submit: My Nguyen <myy@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Commit-Queue: My Nguyen <myy@chromium.org>
Tested-by: My Nguyen <myy@chromium.org>
diff --git a/README.md b/README.md
index 90c926e..d3b5a92 100644
--- a/README.md
+++ b/README.md
@@ -9,8 +9,8 @@
 builds.
 
 [assets]: https://chromium.googlesource.com/chromiumos/platform/assets
-[common-assets package]: https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/master/chromeos-base/common-assets/
+[common-assets package]: https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/HEAD/chromeos-base/common-assets/
 [chromiumos-assets]: https://chromium.googlesource.com/chromiumos/platform/chromiumos-assets
-[chromiumos-assets package]: https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/master/chromeos-base/chromiumos-assets/
+[chromiumos-assets package]: https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/HEAD/chromeos-base/chromiumos-assets/
 [chromeos-assets]: https://chrome-internal.googlesource.com/chromeos/chromeos-assets
-[chromeos-assets package]: https://chrome-internal.googlesource.com/chromeos/overlays/chromeos-overlay/+/master/chromeos-base/chromeos-assets/
+[chromeos-assets package]: https://chrome-internal.googlesource.com/chromeos/overlays/chromeos-overlay/+/HEAD/chromeos-base/chromeos-assets/