Fixed typo in code sample.

Author: erictraut

docs/spec/overload.rst

@@ -196,7 +196,7 @@ programming error and should be reported by type checkers. The purpose of
 this check is to prevent unsoundness of this form::
 
   @overload
-  def is_one(x: Literal[0]) -> Literal[True]: ...
+  def is_one(x: Literal[1]) -> Literal[True]: ...
   @overload
   def is_one(x: int) -> Literal[False]: ...