message: Change code examples in tf.strings.format to use TF 2.x
  The code examples in tf.strings.format still uses 1.x (`tf.compat.v1`)
  which is not appropriate now.
  This PR changes code examples in tf.strings.format to use TF 2.x.
diff: @@ -123,26 +123,20 @@ def string_format(template, inputs, placeholder="{}", summarize=3, name=None):
  Example:
  Formatting a single-tensor template:
  ```python
  - sess = tf.compat.v1.Session()
  - with sess.as_default():
  tensor = tf.range(10)
  formatted = tf.strings.format("tensor: {}, suffix", tensor)
  - out = sess.run(formatted)
  expected = "tensor: [0 1 2 ... 7 8 9], suffix"
  - assert(out.decode() == expected)
  + assert(formatted == expected)
  ```
  Formatting a multi-tensor template:
  ```python
  - sess = tf.compat.v1.Session()
  - with sess.as_default():
  tensor_one = tf.reshape(tf.range(100), [10, 10])
  tensor_two = tf.range(10)
  formatted = tf.strings.format("first: {}, second: {}, suffix",
  (tensor_one, tensor_two))
  - out = sess.run(formatted)
  expected = ("first: [[0 1 2 ... 7 8 9]\n"
  " [10 11 12 ... 17 18 19]\n"
  " [20 21 22 ... 27 28 29]\n"
  @@ -151,7 +145,7 @@ def string_format(template, inputs, placeholder="{}", summarize=3, name=None):
  " [80 81 82 ... 87 88 89]\n"
  " [90 91 92 ... 97 98 99]], second: [0 1 2 ... 7 8 9], suffix")
  - assert(out.decode() == expected)
  + assert(formatted == expected)
  ```
  Args:
  