message: [tf.lite] Update stale doc reference to recommended NDK version
  The recommended version has been r18b for nearly 6 months, so
  update a lingering doc reference to reflect this.
diff: @@ -76,8 +76,8 @@ have it and the Android NDK and SDK installed on your system.
  1. Install the latest version of the [Bazel build system](https://bazel.build/versions/master/docs/install.html).
  2. The Android NDK is required to build the native (C/C++) TensorFlow Lite
  - code. The current recommended version is 17c, which may be found
  - [here](https://developer.android.com/ndk/downloads/older_releases.html#ndk-17c-downloads).
  + code. The current recommended version is 18b, which may be found
  + [here](https://developer.android.com/ndk/downloads/older_releases.html#ndk-18b-downloads).
  3. The Android SDK and build tools may be obtained
  [here](https://developer.android.com/tools/revisions/build-tools.html), or
  alternatively as part of
  