Doc: corrected reference to customwidget.pro paragraph

Referred to par 1. Corrected to 3.

Task-number: QTBUG-29101
Change-Id: Icaa0e4eb06f24fcc7f2d369401fa8f49ed3b1d29
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>

src/designer/src/designer/doc/src/designer-manual.qdoc

@@ -2534,7 +2534,7 @@ pixmap property in the property editor.
     with specific plugin support for \QD. This is done with the following
     declarations:
 
-    \snippet customwidgetplugin/customwidgetplugin.pro 1
+    \snippet customwidgetplugin/customwidgetplugin.pro 3
 
     If Qt is configured to build in both debug and release modes, \QD will be
     built in release mode. When this occurs, it is necessary to ensure that