Remove WIP tag from SNI

I said I'd put in a PR that does this but I lied, it's basic useful advice so yeah I'm just removing it here.
See also ircv3/ircv3-specifications#331

Author: DanielOaks

docs/sni.md

@@ -1,7 +1,6 @@
 ---
 title: Server Name Indication
 layout: spec
-work-in-progress: true
 redirect_from:
   - /specs/core/sni-3.3.html
 copyrights: