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

@@ -296,14 +296,14 @@ is executed.  If there is a saved exception, it is re-raised at the end of the
 exception or executes a :keyword:`return` or :keyword:`break` statement, the
 saved exception is discarded::
 
-    def f():
-        try:
-            1/0
-        finally:
-            return 42
-
-    >>> f()
-    42
+   >>> def f():
+   ...     try:
+   ...         1/0
+   ...     finally:
+   ...         return 42
+   ...
+   >>> f()
+   42
 
 The exception information is not available to the program during execution of
 the :keyword:`finally` clause.