Imprecise reference to example corrected

Modified one line of descriptive text.

The text refers to "our example," despite there being five other examples in between the reference and the actual example to which the reference is intended.

I added the word "previous" and parentheses with the text "first example on Class Objects," where "Class Objects" is a link to that section.

Author: gabriel-1607

Doc/tutorial/classes.rst

@@ -344,7 +344,7 @@ function that "belongs to" an object.
 
 Valid method names of an instance object depend on its class.  By definition,
 all attributes of a class that are function  objects define corresponding
-methods of its instances.  So in our example, ``x.f`` is a valid method
+methods of its instances.  So in our previous example (first example in :ref:`tut-classobjects`), ``x.f`` is a valid method
 reference, since ``MyClass.f`` is a function, but ``x.i`` is not, since
 ``MyClass.i`` is not.  But ``x.f`` is not the same thing as ``MyClass.f`` --- it
 is a *method object*, not a function object.