Correct -re argument in example readme

[LazerLotus]

According to the ffmpeg document(https://ffmpeg.org/ffmpeg.html), the -re argument is an input option, not a global argument .

-re (input)
Read input at native frame rate. This is equivalent to setting -readrate 1.

The example in the readme ## Stream from a local video to HTTP server part code is not work, so I made a tiny correction.