Documentation: update RPCSEC_GSSv3 RFC link

This draft is an official RFC now.
Signed-off-by: NJ. Bruce Fields <bfields@redhat.com>

Documentation/filesystems/nfs/rpc-server-gss.rst

@@ -13,10 +13,9 @@ RPCGSS is specified in a few IETF documents:
  - RFC2203 v1: https://tools.ietf.org/rfc/rfc2203.txt
  - RFC5403 v2: https://tools.ietf.org/rfc/rfc5403.txt
 
-and there is a 3rd version  being proposed:
+There is a third version that we don't currently implement:
 
- - https://tools.ietf.org/id/draft-williams-rpcsecgssv3.txt
-   (At draft n. 02 at the time of writing)
+ - RFC7861 v3: https://tools.ietf.org/rfc/rfc7861.txt
 
 Background
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