libvpx: update version in README.chromium

v1.8.1 was released and merged over the summer.

Change-Id: I0ec41df84add1e0a78783e45aee14560762a6f68
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1771828
Auto-Submit: Johann Koenig <johannkoenig@google.com>
Reviewed-by: James Zern <jzern@google.com>
Commit-Queue: Johann Koenig <johannkoenig@google.com>
Cr-Commit-Position: refs/heads/master@{#691277}

third_party/libvpx/README.chromium

 Name: libvpx
 URL: http://www.webmproject.org
-Version: v1.8.0
+Version: v1.8.1
 License: BSD
 License File: source/libvpx/LICENSE
 Security Critical: yes