[pygobject] Fix typo in BoundSignal disconnect

[Christoph Reiter <creiter src gnome org>]

commit 506c4c0dbc72f8cc7408c09ebc19b742ec042637
Author: cl0ne <cl0ne mithril org ua>
Date:   Sat Apr 7 00:43:27 2018 +0300

    Fix typo in BoundSignal disconnect

 gi/_signalhelper.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/gi/_signalhelper.py b/gi/_signalhelper.py
index 29d36ea0..6d1f9369 100644
--- a/gi/_signalhelper.py
+++ b/gi/_signalhelper.py
@@ -115,7 +115,7 @@ class Signal(str):
 
         def disconnect(self, handler_id):
             """Same as GObject.Object.disconnect."""
-            self.instance.disconnect(handler_id)
+            self.gobj.disconnect(handler_id)
 
         def emit(self, *args, **kargs):
             """Same as GObject.Object.emit except there is no need to specify