Fix docs for the record command

[jmargeta]

Renames the non-existent "render" command to "record"

[yuvaltassa]

Thanks!

Obviously this change is good, are there any lessons from the discussion in #127 that we want to incorporate in this PR? cc @kevinzakka

[jmargeta]

Great, thanks for the pointer @yuvaltassa Agree, I updated ffmpeg's arguments to work well with the humanoid example.

There is always a black rectangle on the bottom left which should be the "subsampled depth image" https://github.com/google-deepmind/mujoco/blob/a00f1b5f3043ed1cf676b130a15a2b2ee7700db3/sample/record.cc#L278

I would push one more change to optionally disable the depth map overlay through a command line arg, e.g.: record humanoid.xml 5 60 rgb.out 0

[jmargeta]

With pleasure! I think it is a much better PR than it originally was and I got to learn something new about offscreen rendering as a bonus, so thank you @yuvaltassa =)