Fix typo error tf.strings.substr

Replaced tf.substr with proper API name tf.strings.substr

Author: gadagashwini

site/en/guide/ragged_tensor.ipynb

@@ -109,7 +109,7 @@
       "source": [
         "### What you can do with a ragged tensor\n",
         "\n",
-        "Ragged tensors are supported by more than a hundred TensorFlow operations, including math operations (such as `tf.add` and `tf.reduce_mean`), array operations (such as `tf.concat` and `tf.tile`), string manipulation ops (such as `tf.substr`), control flow operations (such as `tf.while_loop` and `tf.map_fn`), and many others:"
+        "Ragged tensors are supported by more than a hundred TensorFlow operations, including math operations (such as `tf.add` and `tf.reduce_mean`), array operations (such as `tf.concat` and `tf.tile`), string manipulation ops (such as `tf.strings.substr`), control flow operations (such as `tf.while_loop` and `tf.map_fn`), and many others:"
       ]
     },
     {