docs: fix 'make htmldocs' warning in SCTP.rst

Fix following 'make htmldocs' warnings:
./Documentation/security/SCTP.rst:123: WARNING: Title underline too short.
security_sctp_assoc_established()
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
./Documentation/security/SCTP.rst:123: WARNING: Title underline too short.
security_sctp_assoc_established()
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
./Documentation/security/SCTP.rst:273: WARNING: Title underline too short.
security_sctp_assoc_established()
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
./Documentation/security/SCTP.rst:273: WARNING: Title underline too short.
security_sctp_assoc_established()
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Fixes: 5e50f5d4 ("security: add sctp_assoc_established hook")
Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Reviewed-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

Documentation/security/SCTP.rst

@@ -120,7 +120,7 @@ calls **sctp_peeloff**\(3).
 
 
 security_sctp_assoc_established()
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Called when a COOKIE ACK is received, and the peer secid will be
 saved into ``@asoc->peer_secid`` for client::
 
@@ -270,7 +270,7 @@ sockets sid and peer sid to that contained in the ``@asoc sid`` and
 
 
 security_sctp_assoc_established()
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Called when a COOKIE ACK is received where it sets the connection's peer sid
 to that in ``@skb``::