message: Update kernel_methods to fix failing build_docs_test
diff: # Improving Linear Models Using Explicit Kernel Methods
  Note: This document uses a deprecated version of @{tf.estimator},
  -which has a @{tf.contrib.learn.estimator$different interface}.
  +which has a different interface (see `tf.contrib.learn Estimator`).
  It also uses other `contrib` methods whose
  @{$version_compat#not_covered$API may not be stable}.
  