cpufreq-stats: Minor documentation fix

The cpufreq-stats code can no longer be built as a module, so it now
appears with square brackets in menuconfig.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Fixes: 1aefc75b (cpufreq: stats: Make the stats code non-modular)
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>