docs: hwmon: Add an entry for mp2888

The entry for mp2888 is missing and it causes the following
'make htmldocs' build warning:

Documentation/hwmon/mp2888.rst: WARNING: document isn't included in any toctree

Add the mp2888 entry.
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Link: https://lore.kernel.org/r/20210521172218.37592-1-festevam@gmail.comSigned-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/hwmon/index.rst

@@ -137,6 +137,7 @@ Hardware Monitoring Kernel Drivers
    mcp3021
    menf21bmc
    mlxreg-fan
+   mp2888
    mp2975
    nct6683
    nct6775