doc: Fix variable name in QDBusArgument snippet

Change-Id: I751802d3f9f389e80cbb9aabcdb637d4742d3832
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/dbus/doc/snippets/code/src_qdbus_qdbusargument.cpp

@@ -186,7 +186,7 @@ const QDBusArgument &operator>>(const QDBusArgument &argument, MyDictionary &myd
     argument.beginMap();
     mydict.clear();
 
-    while ( !argMap.atEnd() ) {
+    while ( !argument.atEnd() ) {
         int key;
         MyValue value;
         argument.beginMapEntry();