Documentation: KVM: extend KVM_CAP_VM_DISABLE_NX_HUGE_PAGES heading underline

Extend heading underline for KVM_CAP_VM_DISABLE_NX_HUGE_PAGE to match
the heading text length.

Link: https://lore.kernel.org/lkml/20220627181937.3be67263@canb.auug.org.au/
Fixes: 084cc29f

 ("KVM: x86/MMU: Allow NX huge pages to be disabled on a per-vm basis")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: David Matlack <dmatlack@google.com>
Cc: Ben Gardon <bgardon@google.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: kvm@vger.kernel.org
Cc: linux-next@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Message-Id: <20220627095151.19339-2-bagasdotme@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>