(doc) Adapt mpv process example in player setup

[jreusch]

Cli flag was removed in newer mpv versions. Was already adapted in raw mpv example but not in process input one.

example given for process mpv produced

Option ao: this option does not accept sub-options.
Sub-options for --vo and --ao were removed from mpv in release 0.23.0.
See https://0x0.st/uM for details.
Error parsing option ao (option parameter could not be parsed)
Setting commandline option --ao=pcm:file=/dev/stdout failed.

Took us five minutes to figure this out, hope this safes others some time. :)
When changing saw that up in the file the explicit mpv example was already adapted.

[jreusch]

gnah, dropped one dash in the argument. added&verified finally.

[badaix]

Thanks!