[webauthn] "If extensions was specified" should instead use the "present" terminology

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

== "If extensions was specified" should instead use the "present" 
terminology ==
https://w3c.github.io/webauthn/#dom-webauthentication-makecredential 
step 7 should start with something like:

> If the extensions member of _options_ is present, then process its 
value ....

with "present" ideally linking to 
https://heycam.github.io/webidl/#dfn-present



Please view or discuss this issue at 
https://github.com/w3c/webauthn/issues/269 using your GitHub account