Add prompts to interactive example.

This makes it match the new example below, and allows Sphinx's
"hide the prompts and output" feature to work.

Doc/reference/compound_stmts.rst

@@ -313,12 +313,12 @@ exception, the saved exception is set as the context of the new exception.
 If the :keyword:`finally` clause executes a :keyword:`return` or :keyword:`break`
 statement, the saved exception is discarded::
 
-    def f():
-        try:
-            1/0
-        finally:
-            return 42
-
+   >>> def f():
+   ...     try:
+   ...         1/0
+   ...     finally:
+   ...         return 42
+   ...
    >>> f()
    42