updated the definition of authenticator (#607)

* updated the definition of authenticator

* updated according to comment

index.bs

@@ -283,10 +283,10 @@ The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "S
     [=authenticatorGetAssertion=] operation.
 
 : <dfn>Authenticator</dfn>
-:: A cryptographic device used by a [=[WAC]=] to (i) generate a [=public key credential=] and register it with a [=[RP]=],
-    and (ii) subsequently used to cryptographically sign and return, in the form of an [=Authentication Assertion=], a
-    challenge and other data presented by a [=[RP]=] (in concert with the [=[WAC]=]) in order to effect
-    [=authentication=].
+:: A cryptographic entity used by a [=[WAC]=] to (i) generate a [=public key credential=] and register it with a [=[RP]=],
+    and (ii) [=authentication|authenticate=] by potentially [=user verification|verifying the user=], and then 
+    cryptographically signing and returning, in the form of an [=Authentication Assertion=], 
+    a challenge and other data presented by a [=[RP]=] (in concert with the [=[WAC]=]).
 
 : <dfn>Authorization Gesture</dfn>
 :: An [=authorization gesture=] is a physical interaction performed by a user with an authenticator as part of a [=ceremony=],