Fix typo in the text.

Author: mykola-radionov

docs/readability/test_for_object_identity_should_be_is_not.rst

@@ -2,7 +2,7 @@ Test for object identity should be ``is``
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 Testing the identity of two objects can be achieved in python with a special operator called ``is``.
-Most prominently it is used to check whether an variable points to ``None``.
+Most prominently it is used to check whether a variable points to ``None``.
 But the operator can examine any kind of identity.
 This often leads to confusion because equality of two different objects will return ``False``.