#3040: include 'dest' argument in example; trim some trailing whitespace

Doc/library/optparse.rst

@@ -1637,7 +1637,7 @@ arguments::
        setattr(parser.values, option.dest, value)
 
    [...]
-   parser.add_option("-c", "--callback",
+   parser.add_option("-c", "--callback", dest="vararg_attr",
                      action="callback", callback=vararg_callback)
 
 The main weakness with this particular implementation is that negative numbers