Fix sys.stderr typo.

pep-0389.txt

@@ -244,15 +244,15 @@ but are out of the scope of this PEP, and have been redirected to
 the argparse issue tracker.
 
 
-Discussion: sys.err and sys.exit
-================================
+Discussion: sys.stderr and sys.exit
+===================================
 There were some concerns that argparse by default always writes to
-``sys.err`` and always calls ``sys.exit`` when invalid arguments are
-provided. This is the desired behavior for the vast majority of
+``sys.stderr`` and always calls ``sys.exit`` when invalid arguments
+are provided. This is the desired behavior for the vast majority of
 argparse use cases which revolve around simple command line
 interfaces. However, in some cases, it may be desirable to keep
 argparse from exiting, or to have it write its messages to something
-other than ``sys.err``. These use cases can be supported by
+other than ``sys.stderr``. These use cases can be supported by
 subclassing ``ArgumentParser`` and overriding the ``exit`` or
 ``_print_message`` methods. The latter is an undocumented
 implementation detail, but could be officially exposed if this turns