update ffmpeg syntax for the -codec switch (#9927)

as far as i can tell, the syntax does not use `:video` and `:audio` for the stream specifier, and instead uses `:v` and `:a`, respectively

i'm on version 4.4.2, on ubuntu

`ffmpeg is already the newest version (7:4.4.2-0ubuntu0.22.04.1)`

pages/common/ffmpeg.md

@@ -25,12 +25,12 @@
 
 - Convert AVI video to MP4. AAC Audio @ 128kbit, h264 Video @ CRF 23:
 
-`ffmpeg -i {{input_video}}.avi -codec:audio aac -b:audio 128k -codec:video libx264 -crf 23 {{output_video}}.mp4`
+`ffmpeg -i {{input_video}}.avi -codec:a aac -b:a 128k -codec:v libx264 -crf 23 {{output_video}}.mp4`
 
 - Remux MKV video to MP4 without re-encoding audio or video streams:
 
 `ffmpeg -i {{input_video}}.mkv -codec copy {{output_video}}.mp4`
 
-- Convert MP4 video to VP9 codec. For the best quality, use a CRF value (recommended range 15-35) and -b:video MUST be 0:
+- Convert MP4 video to VP9 codec. For the best quality, use a CRF value (recommended range 15-35) and -b:v MUST be 0:
 
-`ffmpeg -i {{input_video}}.mp4 -codec:video libvpx-vp9 -crf {{30}} -b:video 0 -codec:audio libopus -vbr on -threads {{number_of_threads}} {{output_video}}.webm`
+`ffmpeg -i {{input_video}}.mp4 -codec:v libvpx-vp9 -crf {{30}} -b:v 0 -codec:a libopus -vbr on -threads {{number_of_threads}} {{output_video}}.webm`