message: Fix broken link in kernel method tutorial
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}.
  +Note: This document uses a deprecated version of @{tf.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}.
  +@{$version_compat#not_covered$API may not be stable}.
  In this tutorial, we demonstrate how combining (explicit) kernel methods with
  linear models can drastically increase the latters' quality of predictions
  