Fix documentation of `flameshot gui --help` (#2949)

In
https://github.com/flameshot-org/flameshot/commit/6432490c316baa89fe3f5a6910539a1157614577,
the format of `--print-geometry` changed from `W H X Y` to `WxH+X+Y`,
but it looks like we forgot to update the docs accordingly.