docs: :pencil: fix set-fps example on README.md

[MauroSoli]

Otherwise, the command does not work:

v4l2loopback-ctl -h | grep set-fps set-fps /dev/videoX Set device fps

[umlaeute]

your updates seem to apply to (and target) a different version of v4l2loopback. at least: the v4l2loopback utility built with make in current master (b261f19bc74862abb3ad1999bd8e8ba69052b105) has the syntax v4l2loopback-ctl <device> <fps>

$ ./utils/v4l2loopback-ctl -h |& grep set-fps
     set-fps <device> <fps>
 setting framerate ('set-fps')
    ./utils/v4l2loopback-ctl set-fps <device> <fps>

$ ./utils/v4l2loopback-ctl set-fps 30 /dev/video2 
WARNING: './utils/v4l2loopback-ctl set-fps <fps> <image>' is deprecated!
WARNING: use './utils/v4l2loopback-ctl set-fps <device> <fps>' instead.

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