Issue #12160: Fix incorrect StreamCodec references in Codec.encode() and Codec.decode() docs.

It should StreamWriter for Codecs.encode() and StreamReader for Codec.decode().

Patch by Nick Weinhold.
diff --git a/Doc/library/codecs.rst b/Doc/library/codecs.rst
index 19d7192..9496035 100644
--- a/Doc/library/codecs.rst
+++ b/Doc/library/codecs.rst
@@ -436,8 +436,8 @@
    It defaults to ``'strict'`` handling.
 
    The method may not store state in the :class:`Codec` instance. Use
-   :class:`StreamCodec` for codecs which have to keep state in order to make
-   encoding/decoding efficient.
+   :class:`StreamWriter` for codecs which have to keep state in order to make
+   encoding efficient.
 
    The encoder must be able to handle zero length input and return an empty object
    of the output object type in this situation.
@@ -458,8 +458,8 @@
    It defaults to ``'strict'`` handling.
 
    The method may not store state in the :class:`Codec` instance. Use
-   :class:`StreamCodec` for codecs which have to keep state in order to make
-   encoding/decoding efficient.
+   :class:`StreamReader` for codecs which have to keep state in order to make
+   decoding efficient.
 
    The decoder must be able to handle zero length input and return an empty object
    of the output object type in this situation.