Documentation: xfrm_sync: Trim excess section heading characters

Documentation: xfrm_sync: Trim excess section heading characters

The first section "Message Structure" has excess underline, while the
second and third one ("TLVS reflect the different parameters" and
"Default configurations for the parameters") have trailing colon. Trim
them.

Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Suggested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>