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>