Typo on "allowusermedia" section

[compulim]

In allowusermedia, there is a copy-and-paste typo:

If the iframe element has an "allow" attribute whose value contains the token "payment", then the "allowusermedia" attribute must have no effect.

Instead of payment, it should be "camera or microphone".

[clelland]

Thanks -- it actually needs to be a bit more verbose than the other sections, since the intention is that "allowusermedia" is equivalent to allow="camera *; microphone *", except that specifying either camera or microphone will override the policy for just that feature, leaving the other untouched.

I'll see if I can reword it to capture that.

[foolip]

@clelland looks like this wasn't automatically closed by #164.