Fix punctuation.

Doc/tutorial/classes.rst

@@ -537,7 +537,7 @@ Private Variables
 =================
 
 "Private" instance variables that cannot be accessed except from inside an
-object, don't exist in Python.  However, there is a convention that is followed
+object don't exist in Python.  However, there is a convention that is followed
 by most Python code: a name prefixed with an underscore (e.g. ``_spam``) should
 be treated as a non-public part of the API (whether it is a function, a method
 or a data member).  It should be considered an implementation detail and subject