tools: cpupower: fix typo in cpupower-idle-set(1) manpage

The tools name was wrong in the SYNTAX section of the manpage it should
read "idle-set" instead of "idle-info".

Signed-off-by: Andreas Rammhold <andreas@rammhold.de>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>