Documentation: vmcoreinfo: Fix htmldocs warning

Since commit 2369f171 ("arm64: crash_core: Export MODULES, VMALLOC,
and VMEMMAP ranges"), Stephen reports a warning when building htmldocs:

  | Documentation/admin-guide/kdump/vmcoreinfo.rst:498: WARNING: Title underline too short.

Extend the underline to squash the warning.

Fixes: 2369f171 ("arm64: crash_core: Export MODULES, VMALLOC, and VMEMMAP ranges")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Will Deacon <will@kernel.org>

Documentation/admin-guide/kdump/vmcoreinfo.rst

@@ -495,7 +495,7 @@ addresses in the higher VA range (refer to ARMv8 ARM document for
 more details).
 
 MODULES_VADDR|MODULES_END|VMALLOC_START|VMALLOC_END|VMEMMAP_START|VMEMMAP_END
--------------
+-----------------------------------------------------------------------------
 
 Used to get the correct ranges:
 	MODULES_VADDR ~ MODULES_END-1 : Kernel module space.