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