Fix typo in Python tutorial

Doc/tutorial/classes.rst

@@ -120,7 +120,7 @@ are directly accessible:
 If a name is declared global, then all references and assignments go directly to
 the middle scope containing the module's global names.  To rebind variables
 found outside of the innermost scope, the :keyword:`nonlocal` statement can be
-used; if not declared nonlocal, those variable are read-only (an attempt to
+used; if not declared nonlocal, those variables are read-only (an attempt to
 write to such a variable will simply create a *new* local variable in the
 innermost scope, leaving the identically named outer variable unchanged).