[pygobject] docs: fix build with sphinx 1.7.0

[Christoph Reiter <creiter src gnome org>]

commit 3e91df3dca3691635b978d1dc661506213e0efe3
Author: Christoph Reiter <reiter christoph gmail com>
Date:   Mon Feb 12 15:39:41 2018 +0100

    docs: fix build with sphinx 1.7.0
    
    1.7.0 no longer allows non-class references as param types

 docs/guide/api/signals.rst | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/docs/guide/api/signals.rst b/docs/guide/api/signals.rst
index 00018625..4353b21a 100644
--- a/docs/guide/api/signals.rst
+++ b/docs/guide/api/signals.rst
@@ -63,7 +63,8 @@ You can define your own signals using the :obj:`GObject.Signal` decorator:
     :param GObject.SignalFlags flags: Signal flags
     :param GObject.GType return_type: Return type
     :param list arg_types: List of :class:`GObject.GType` argument types
-    :param GObject.SignalAccumulator accumulator: Accumulator function
+    :param accumulator: Accumulator function
+    :type accumulator: :obj:`GObject.SignalAccumulator`
     :param object accu_data: User data for the accumulator