add period

Co-authored-by: Michael B. Jones <[email protected]>

index.bs

@@ -463,7 +463,7 @@ A temporary PIN was sent to the user out of band (ex: via an RCS message).
 - On a laptop or desktop:
     * User navigates to corp.example.com in a browser and initiates signing in.
     * The browser or operating system prompts the user for their security key.
-    * The user connects their USB security key
+    * The user connects their USB security key.
     * The USB security key blinks to indicate the user should press the button on it; the user does.
     * The browser or operating system asks the user to enter their PIN.
     * The user enters the temporary PIN they were provided and continues.