Switch intended status to match the draft charter

I just copy-pasted this from anoher document without thinking about this
too hard two years ago.

Author: davidben

draft-davidben-tls-merkle-tree-certs.md

@@ -2,7 +2,7 @@
 title: Merkle Tree Certificates
 docname: draft-davidben-tls-merkle-tree-certs-latest
 submissiontype: IETF
-category: exp
+category: std
 number:
 date:
 consensus: true