Fix Android NDK links.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=120107182

extensions/opus/README.md

@@ -26,6 +26,8 @@ OPUS_EXT_PATH="${EXOPLAYER_ROOT}/extensions/opus/src/main"
 
 * Download the [Android NDK][] and set its location in an environment variable:
 
+[Android NDK]: https://developer.android.com/tools/sdk/ndk/index.html
+
 ```
 NDK_PATH="<path to Android NDK>"
 ```

extensions/vp9/README.md

@@ -26,6 +26,8 @@ VP9_EXT_PATH="${EXOPLAYER_ROOT}/extensions/vp9/src/main"
 
 * Download the [Android NDK][] and set its location in an environment variable:
 
+[Android NDK]: https://developer.android.com/tools/sdk/ndk/index.html
+
 ```
 NDK_PATH="<path to Android NDK>"
 ```