docs: trace/index.rst: add histogram-design.rst

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: NDaniel W. S. Almeida <dwlsalmeida@gmail.com>
Link: https://lore.kernel.org/r/20200718165107.625847-12-dwlsalmeida@gmail.comSigned-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/trace/index.rst

@@ -20,6 +20,7 @@ Linux Tracing Technologies
    events-msr
    mmiotrace
    histogram
+   histogram-design
    boottime-trace
    hwlat_detector
    intel_th