Update page title block name on signed out page

Now matches the block from the GOVUKFrontend
template.

This is the only page not suffixed by "GOV.UK
Notify" so has it's specific content for this
block.

app/templates/views/signedout.html

@@ -4,7 +4,7 @@
   <meta name="description" content="GOV.UK Notify lets you send emails, text messages and letters to your users. Try it now if you work in central government, a local authority, or the NHS.">
 {% endblock %}
 
-{% block page_title %}
+{% block pageTitle %}
   GOV.UK Notify
 {% endblock %}