message: Update docstring for tf.linalg.matvec
  In tensorflow, `matvec` is exposed as `tf.linalg.matvec`
  but in the docstring `tf.matvec` is used instead.
  This fix fixes the docstring `tf.matvec` => `tf.linalg.matvec`
  This fix fixes 31923.
diff: @@ -2733,7 +2733,7 @@ def matvec(a,
  # `a` * `b`
  # [ 58, 64]
  - c = tf.matvec(a, b)
  + c = tf.linalg.matvec(a, b)
  # 3-D tensor `a`
  @@ -2753,7 +2753,7 @@ def matvec(a,
  # `a` * `b`
  # [[ 86, 212],
  # [410, 563]]
  - c = tf.matvec(a, b)
  + c = tf.linalg.matvec(a, b)
  ```
  Args:
  