[mediacapture-image] Possibly incorrect note on focusMode (#293)

JohnHardy has just created a new issue for https://github.com/w3c/mediacapture-image:

== Possibly incorrect note on focusMode ==
In "_10. Photo Capabilities and Constrainable Properties_" point 3 it says "_Focus mode describes the focus setting of the capture device (e.g. `auto` or `manual`)._"

However, focusMode, of type sequence<DOMString> is defined as "A sequence of supported focus modes. Each string MUST be one of the members of `MeteringMode`."  Of which the values `none`, `manual`, `single-shot`, or `continuous`

To fix this issue, we could instead link to the `MeteringMode` enum.

Please view or discuss this issue at https://github.com/w3c/mediacapture-image/issues/293 using your GitHub account


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