Correct mistake in docs about the supported ambisonics order in Unity

[Schroedingers-Cat]

Steam audio docs falsely state that Unity supports 2nd order ambisonics clips. While that would be great indeed, it doesn't appear to be the case (tested in 2020.3 and 2021.1 on macOS, playback fails within Unity's fmod library due to the channel count being above 8). Also, the Unity docs linked to in the guide clearly state the opposite:

There is no support for second order ambisonics.

This PR fixes this mistake.