bpo-39869: Fix typo in 'Instance objects' section. (GH-18889)



diff --git a/Doc/tutorial/classes.rst b/Doc/tutorial/classes.rst
index f781fec..06bdd0d 100644
--- a/Doc/tutorial/classes.rst
+++ b/Doc/tutorial/classes.rst
@@ -323,7 +323,7 @@
 
 Now what can we do with instance objects?  The only operations understood by
 instance objects are attribute references.  There are two kinds of valid
-attribute names, data attributes and methods.
+attribute names: data attributes and methods.
 
 *data attributes* correspond to "instance variables" in Smalltalk, and to "data
 members" in C++.  Data attributes need not be declared; like local variables,