Add screenshot to clarify how to 'administration password' manifests itself (#15571)

wiki/src/doc/first_steps/startup_options/administration_password.mdwn

@@ -13,6 +13,8 @@ This can prevent an attacker with physical or remote access to your Tails system
 to gain administration privileges and perform administration tasks
 against your will.
 
+[[!img password-prompt.png link="no" alt="Authentication required: amnesia password"]]
+
 Set up an administration password
 =================================