Add the word "OPTIONAL" to userVerification field to match surround…

…ing style.

Previously, `userVerification` was the only optional field of `PublicKeyCredentialRequestOptions` that didn't include the word "OPTIONAL", which can suggest at first glance that this field is not optional like the other optional ones. Adding the word is simple, and should avoid potential confusion.

index.bs

@@ -2145,7 +2145,7 @@ an assertion. Its {{PublicKeyCredentialRequestOptions/challenge}} member MUST be
         preferred credential, and so on down the list).
 
     :   <dfn>userVerification</dfn>
-    ::  This member describes the [=[RP]=]'s requirements regarding [=user verification=] for the
+    ::  This OPTIONAL member describes the [=[RP]=]'s requirements regarding [=user verification=] for the
         {{CredentialsContainer/get()}} operation. Eligible authenticators are filtered to only those capable of satisfying this
         requirement.