Fixed description of num_threads in `tf.lite.interpreter` doc
Hi
As per the code  [tensorflow/tensorflow/lite/python/interpreter.py](https://github.com/tensorflow/tensorflow/blob/b297295711589c2d71880a046333e6ec095c6d7d/tensorflow/lite/python/interpreter.py#L460-L464)

Lines 460 to 464 in [b297295](https://github.com/tensorflow/tensorflow/commit/b297295711589c2d71880a046333e6ec095c6d7d) , code num_threads is greater than equal to 1 but in description it was given as `-1`. This PR fixes description of num_threads in `tf.lite.interpreter` doc.

This PR fixes: TFLite issue [#68862](https://github.com/tensorflow/tensorflow/issues/68862)

Thank You