Documentation: fix sysctl/kernel.rst heading format warnings

Fix heading format warnings in admin-guide/sysctl/kernel.rst:

Documentation/admin-guide/sysctl/kernel.rst:339: WARNING: Title underline too short.
hung_task_all_cpu_backtrace:
================

Documentation/admin-guide/sysctl/kernel.rst:650: WARNING: Title underline too short.
oops_all_cpu_backtrace:
================

Fixes: 0ec9dc9b ("kernel/hung_task.c: introduce sysctl to print all traces when a hung task is detected")
Fixes: 60c958d8 ("panic: add sysctl to dump all CPUs backtraces on oops event")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/8af1cb77-4b5a-64b9-da5d-f6a95e537f99@infradead.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/sysctl/kernel.rst

@@ -335,8 +335,8 @@ Path for the hotplug policy agent.
 Default value is "``/sbin/hotplug``".
 
 
-hung_task_all_cpu_backtrace:
-================
+hung_task_all_cpu_backtrace
+===========================
 
 If this option is set, the kernel will send an NMI to all CPUs to dump
 their backtraces when a hung task is detected. This file shows up if
@@ -646,8 +646,8 @@ rate for each task.
 scanned for a given scan.
 
 
-oops_all_cpu_backtrace:
-================
+oops_all_cpu_backtrace
+======================
 
 If this option is set, the kernel will send an NMI to all CPUs to dump
 their backtraces when an oops event occurs. It should be used as a last