Fix typos in libgav1 README: libvpx => libgav1.

Bug: 1026522
Change-Id: I91536620b13ba571a701cb0c1931f1f49a2bc625
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2037730Reviewed-by: James Zern <jzern@google.com>
Commit-Queue: James Zern <jzern@google.com>
Commit-Queue: Wan-Teh Chang <wtc@google.com>
Cr-Commit-Position: refs/heads/master@{#738302}

third_party/libgav1/README.chromium

@@ -18,11 +18,11 @@ Please follow these steps to update libgav1 src code
   roll_dep.py \
   -r <libgav1 OWNER> \
   --log-limit 20 \
-  --roll-to <libvpx hash> \
+  --roll-to <libgav1 hash> \
   src/third_party/libgav1/src
 
   Use the generated commit message for the roll.
 
 2. Generate .gni and update Date and Commit in README.chromium
-  cd third_party/libvpx
+  cd third_party/libgav1
   go run generate_libgav1_src_gni.go