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Fix the following warning:
Documentation/trace/histogram-design.rst:
WARNING: document isn't included in any toctreeBy adding 'histogram-design.rst' to the index.
Signed-off-by: Daniel W. S. Almeida
Link: https://lore.kernel.org/r/20200718165107.625847-12-dwlsalmeida@gmail.com
Signed-off-by: Jonathan Corbet