Slight tidy in how immutability works docs (#419)

docs/how-does-it-work.rst

@@ -53,7 +53,7 @@ Immutability
 
 In order to give you immutability, ``attrs`` will attach a ``__setattr__`` method to your class that raises a :exc:`attr.exceptions.FrozenInstanceError` whenever anyone tries to set an attribute.
 
-Depending on whether of not a class is a dict class or a slots class, ``attrs`` uses a different technique to circumvent that limitation in the ``__init__`` method.
+Depending on whether a class is a dict class or a slots class, ``attrs`` uses a different technique to circumvent that limitation in the ``__init__`` method.
 
 Once constructed, frozen instances don't differ in any way from regular ones except that you cannot change its attributes.