[gtkmm] Builder: Use refBuilder-> not refXml-> in examples

[Daniel Boles <dboles src gnome org>]

commit ef4b55558c0fda8c49708d141f1fe4be0f7d8f27
Author: Daniel Boles <dboles src gnome org>
Date:   Sun Dec 16 23:46:26 2018 +0000

    Builder: Use refBuilder-> not refXml-> in examples
    
    It's much less clear what refXml is when we could just say refBuilder,
    as we already do elsewhere.

 gtk/src/builder.hg | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/gtk/src/builder.hg b/gtk/src/builder.hg
index 889d3e4b..1ca53b5f 100644
--- a/gtk/src/builder.hg
+++ b/gtk/src/builder.hg
@@ -445,7 +445,7 @@ public:
    * For instance:
    * @code
    * Gtk::Grid* pGrid = nullptr;
-   * refXml->get_widget("mygrid", pGrid);
+   * refBuilder->get_widget("mygrid", pGrid);
    * @endcode
    * This method prints a warning message to the console if the widget
    * doesn't exist or has the wrong type, so you don't need to check that