[webauthn] UVM Extension Editorial Change

apowers313 has just created a new issue for 
https://github.com/w3c/webauthn:

== UVM Extension Editorial Change ==
The [UVM Extension](https://w3c.github.io/webauthn/#uvm-extension) 
text currently includes the following:

> **Authenticator processing**
The authenticator augments the authenticator data with a user 
verification index indicating the method used by the user to authorize
 the operation, as defined below. This extension can be added to 
attestation objects and assertions.

Note the mention of `user verification index`, which was probably 
copied from the UVI extension. I'm not sure if the fix is as simple as
 changing `index` to `mode` or if the whole section should be 
re-written.

Please view or discuss this issue at 
https://github.com/w3c/webauthn/issues/368 using your GitHub account