Add __eq__ method to assertion comparison example

This makes the example make more sense, because now Foo(1) == Foo(1) is
true. Fixes #411

doc/en/assert.rst

@@ -202,6 +202,9 @@ now, given this test module::
        def __init__(self, val):
            self.val = val
 
+       def __eq__(self, other):
+           return self.val == other.val
+
    def test_compare():
        f1 = Foo(1)
        f2 = Foo(2)