Fix the following warning:

Documentation/trace/histogram-design.rst:
WARNING: document isn't included in any toctree

By adding 'histogram-design.rst' to the index.
Signed-off-by: Daniel W. S. Almeida <dwlsalmeida@gmail.com>
Link: https://lore.kernel.org/r/20200718165107.625847-12-dwlsalmeida@gmail.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>