bpo-30709: Improve code example in Descriptor HowTo doc (GH-2339)

diff --git a/Doc/howto/descriptor.rst b/Doc/howto/descriptor.rst
index 2dd6c34..b349375 100644
--- a/Doc/howto/descriptor.rst
+++ b/Doc/howto/descriptor.rst
@@ -252,10 +252,10 @@
 
     class Cell(object):
         . . .
-        def getvalue(self, obj):
-            "Recalculate cell before returning value"
+        def getvalue(self):
+            "Recalculate the cell before returning value"
             self.recalc()
-            return obj._value
+            return self._value
         value = property(getvalue)