Prompt for credential selection only if more than one eligible

Per review comment by @rlin1.

index.bs

@@ -2163,7 +2163,19 @@ When this method is invoked, the [=authenticator=] must perform the following pr
 1. If |allowCredentialDescriptorList| is now empty, return an error code equivalent to "{{NotAllowedError}}" and terminate the
     operation.
 
-1. Prompt the user to select a [=public key credential|credential=] |selectedCredential| from |allowCredentialDescriptorList|.
+1. Let |selectedCredential| be a [=public key credential|credential=] as follows. If the [=list/size=] of
+    |allowCredentialDescriptorList|
+
+    <dl class="switch">
+        :   is exactly 1
+        ::  Let |selectedCredential| be the [=public key credential|credential=] matching
+            <code>|allowCredentialDescriptorList|[0]</code>.
+
+        :   is greater than 1
+        ::  Prompt the user to select |selectedCredential| from the [=public key credential|credentials=] matching the
+            [=list/items=] in |allowCredentialDescriptorList|.
+    </dl>
+
 
 1. Obtain [=user consent=] for using |selectedCredential|. The prompt for obtaining this [=user consent|consent=] may be shown by
     the [=authenticator=] if it has its own output capability, or by the user agent otherwise. The prompt SHOULD display the