PEP 483: Fix an example (#2143)

The body of one of the examples mistakenly used different names than the parameters given in the function signature.

pep-0483.txt

@@ -486,7 +486,7 @@ replaced by the most-derived base class among ``t1``, etc. Examples:
   Note that the type checker will reject this function::
 
     def concat(first: U, second: U) -> U:
-        return x + y  # Error: can't concatenate str and bytes
+        return first + second  # Error: can't concatenate str and bytes
 
   For such cases where parameters could change their types only simultaneously
   one should use constrained type variables.