Remove autolink to nonexistent term "WebAuthn credentials"

Fixes this Bikeshed lint:

```
LINE ~8667: No 'dfn' refs found for 'webauthn credentials'.
[=WebAuthn credentials=]
```

index.bs

@@ -8666,7 +8666,7 @@ the [=[RP]=] MUST validate the {{CollectedClientData/origin}} member of the [=cl
 
 The [=[RP]=] MUST NOT accept unexpected values of {{CollectedClientData/origin}},
 as doing so could allow a malicious website to obtain valid [=credentials=].
-Although the [=scope=] of [=WebAuthn credentials=] prevents their use on domains
+Although the [=scope=] of WebAuthn credentials prevents their use on domains
 outside the [=RP ID=] they were registered for,
 the [=[RP]=]'s origin validation serves as an additional layer of protection
 in case a faulty [=authenticator=] fails to enforce credential [=scope=].