Documentation/x86: Fix title underline length

Fix:

  Documentation/arch/x86/resctrl.rst:577: WARNING: Title underline too short.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Tony Luck <tony.luck@intel.com>
Link: https://lore.kernel.org/r/20240325121750.265d655c@canb.auug.org.au