docs: xfrm: Fix the two aevent sysctl paths

The sysctls are registered as xfrm_aevent_etime and xfrm_aevent_rseqth
under net.core (net/xfrm/xfrm_sysctl.c). The document calls them
sysctl_xfrm_aevent_etime and sysctl_xfrm_aevent_rseqth; those names
appear nowhere in the code but in a comment in net/xfrm/xfrm_user.c and
were never the proc file names.

Use the proc file names.

Fixes: b8a99520f78e ("[XFRM]: Add documentation for async events.")
Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>