Fix Client Library Documentation link (#7109)

Was pointed to https://googlecloudplatform.github.io/google-cloud-python/latest/texttospeech/api.html
Which has no content.
The correct URL appears to be https://googlecloudplatform.github.io/google-cloud-python/latest/texttospeech/index.html

packages/google-cloud-texttospeech/README.rst

@@ -16,7 +16,7 @@ powerful neural network models.
 .. |versions| image:: https://img.shields.io/pypi/pyversions/google-cloud-texttospeech.svg
    :target: https://pypi.org/project/google-cloud-texttospeech/
 .. _Cloud Text-to-Speech API: https://cloud.google.com/texttospeech
-.. _Client Library Documentation: https://googlecloudplatform.github.io/google-cloud-python/latest/texttospeech/api.html
+.. _Client Library Documentation: https://googlecloudplatform.github.io/google-cloud-python/latest/texttospeech/index.html
 .. _Product Documentation: https://cloud.google.com/texttospeech
 
 Quick Start