Fix page titles on callbacks documentation

This page is no longer just about inbound.

app/templates/views/callbacks.html

@@ -2,7 +2,7 @@
 {% extends "withoutnav_template.html" %}
 
 {% block per_page_title %}
-  Callbacks for received text messages
+  Callbacks
 {% endblock %}
 
 {% block maincolumn_content %}
@@ -10,7 +10,7 @@
 <div class="grid-row">
   <div class="column-two-thirds">
 
-    <h1 class="heading-large">Callbacks for received text messages</h1>
+    <h1 class="heading-large">Callbacks</h1>
     <p>A callback lets you receive messages from Notify to a URL you choose.</p>
     <p>You’ll need to provide a bearer token, for security. We’ll add this to the authorisation header of the callback request.</p>
     <p>The callback message is in JSON.</p>