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/Lib/codecs.py b/Lib/codecs.py
index 66dd024..4a4d043 100644
--- a/Lib/codecs.py
+++ b/Lib/codecs.py
@@ -141,8 +141,8 @@
             'strict' handling.
 
             The method may not store state in the Codec instance. Use
-            StreamCodec for codecs which have to keep state in order to
-            make encoding/decoding efficient.
+            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
@@ -164,8 +164,8 @@
             'strict' handling.
 
             The method may not store state in the Codec instance. Use
-            StreamCodec for codecs which have to keep state in order to
-            make encoding/decoding efficient.
+            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