message: Fix documentation error in tensorflow feature_utils.h
  Need to take address of the example for modifying its feature
diff: @@ -33,7 +33,7 @@ limitations under the License.
  // GetFeatureValues<int64>("tag", &example)->Add(id);
  //
  // Modification of bytes features is slightly different:
  -// auto tag = GetFeatureValues<string>("tag", example);
  +// auto tag = GetFeatureValues<string>("tag", &example);
  // *tag->Add() = "lorem ipsum";
  //
  // To copy multiple values into a feature:
  