Bump README draft to version 12

Author: fulder

README.rst

@@ -15,7 +15,7 @@ See the original project_, original Python module_, original spec_, and `current
 .. _module: https://github.com/zzsnzmn/py-http-signature
 .. _spec: https://github.com/joyent/node-http-signature/blob/master/http_signing.md
 .. _`current IETF draft`: https://datatracker.ietf.org/doc/draft-cavage-http-signatures/
-.. _`Draft 8`: http://tools.ietf.org/html/draft-cavage-http-signatures-08
+.. _`Draft 12`: http://tools.ietf.org/html/draft-cavage-http-signatures-12
 
 Requirements
 ------------