doctor: display refresh rate with 2 decimal precision

Liu Jierequested to merge
kylinliujie/libkscreen:work/kylinliujie into master

The output of "kscreen-doctor -o" command: Before this changes: image

It was not possible to distinguish between modes 1, 2, 3, and 4 through the output of kscreen-doctor.

After this change: image

Merge request reports