cxl/docs: Fix "Title underline too short" warning

When "Bus" was renamed to "Core" the header underline update was missed.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: 5f653f75 ("cxl/core: Rename bus.c to core.c")
Reviewed-by: Vishal Verma <vishal.l.verma@intel.com>
Link: https://lore.kernel.org/r/162154034053.1995075.17047445540000243300.stgit@dwillia2-desk3.amr.corp.intel.com


Signed-off-by: Dan Williams <dan.j.williams@intel.com>