Update README for libvpx 1.7.0

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=214227045

extensions/vp9/README.md

@@ -49,7 +49,7 @@ git clone https://chromium.googlesource.com/libyuv/libyuv libyuv
 
 
 ```
 ```
 cd "${VP9_EXT_PATH}/jni/libvpx" && \
 cd "${VP9_EXT_PATH}/jni/libvpx" && \
-git checkout tags/v1.6.1 -b v1.6.1 && \
+git checkout tags/v1.7.0 -b v1.7.0 && \
 cd "${VP9_EXT_PATH}/jni/libyuv" && \
 cd "${VP9_EXT_PATH}/jni/libyuv" && \
 git checkout 996a2bbd
 git checkout 996a2bbd
 ```
 ```