Minor docs issue.

Doc/library/stdtypes.rst

@@ -452,7 +452,7 @@ Additional Methods on Integer Types
     Equivalent to::
 
         def bit_length(self):
-            s = bin(x)          # binary representation:  bin(-37) --> '-0b100101'
+            s = bin(self)          # binary representation:  bin(-37) --> '-0b100101'
             s = s.lstrip('-0b') # remove leading zeros and minus sign
             return len(s)       # len('100101') --> 6