docs: Correct inconsistency in GObject tutorial

The example code defines an interface with three methods. The preceding text
reads 'This interface defines two methods'. This appears to be because the
example code was changed without updating the surrounding text.

https://bugzilla.gnome.org/show_bug.cgi?id=790830

docs/reference/gobject/tut_howto.xml

@@ -860,7 +860,7 @@ b_method_to_call (B *obj, gint some_param)
 
   <para>
     The first step is to get the header right. This interface
-    defines two methods:
+    defines three methods:
 <informalexample><programlisting>
 /*
  * Copyright/Licensing information.