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

Author: Ingo Molnar

Documentation/arch/x86/resctrl.rst

@@ -574,7 +574,7 @@ Memory b/w domain is L3 cache.
 	MB:<cache_id0>=bandwidth0;<cache_id1>=bandwidth1;...
 
 Memory bandwidth Allocation specified in MiBps
----------------------------------------------
+----------------------------------------------
 
 Memory bandwidth domain is L3 cache.
 ::