Update the documentation for tf.fft

This fix is related to 17332. While complex128 has been
added in tf, the documentation still says:
```
tf.spectral.fft
input: A Tensor. Must be one of the following types: complex64, complex128. A complex64 tensor.
```

This fix updates the documentation so that it matches the actual behavior.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>