Fix various typos reported to python-docs.

Doc/ref/ref4.tex

@@ -32,7 +32,7 @@ and how execution continues after the code block's execution has
 completed.
 
 A \dfn{scope}\index{scope} defines the visibility of a name within a
-block.  If a local variable is defined in a block, it's scope includes
+block.  If a local variable is defined in a block, its scope includes
 that block.  If the definition occurs in a function block, the scope
 extends to any blocks contained within the defining one, unless a
 contained block introduces a different binding for the name.  The