Use GNOME terminology

wiki/src/install/inc/steps/install_final.inline.mdwn

@@ -95,7 +95,7 @@
    <span class="upgrade">upgrade,</span>
    click on the <span class="button">Install Tails</span> button.
 
-1. Read the warning message in the pop-up window. Click
+1. Read the warning message in the confirmation dialog. Click
    <span class="button">Yes</span> to confirm.
 
    The

wiki/src/upgrade/tails.mdwn

@@ -57,7 +57,7 @@ copied.</p>
 
 1. To start the installation, click on the <span class="button">Install Tails</span> button.
 
-1. Read the warning message in the pop-up window. Click on the <span
+1. Read the warning message in the confirmation dialog. Click on the <span
    class="button">Yes</span> button to confirm.
 
    The installation takes a few minutes.