[FFmpeg-user] Undocumented a Parameter: Disable Data Stream

[ilker tezcan]

Dear Carl,

Thank you for 
https://github.com/FFmpeg/FFmpeg/commit/dcc506a91e7745fc0e4d07e05ff8a13ee75ced41

I added a commit which it is separately in a sentence for -dn option:

https://github.com/FFmpeg/FFmpeg/commit/7ebf8c580732b616e9b6d6f34a75d0c2724271ec

I hope, I wrote/made correctly.
But GitHub can't allow: "You’re editing a file in a project you don’t 
have write access to."