message: fix backquotes
diff: @@ -363,7 +363,7 @@ that will be optimized.
  We can calculate the loss by calling @{tf.losses.sparse_softmax_cross_entropy}.
  The value returned by this function will be approximately 0 at lowest,
  -when the probability of the correct class (at index label') is near 1.0.
  +when the probability of the correct class (at index `label`) is near 1.0.
  The loss value returned is progressively larger as the probability of the
  correct class decreases.
  