docs: fix broken paths to docs/specs/ivshmem-spec.txt

When this file was rewritten/renamed in fdee2025dd,
a reference path was not updated.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

docs/specs/pci-ids.txt

@@ -40,7 +40,7 @@ maintained as part of the virtio specification.
 1af4:1100  Used as PCI Subsystem ID for existing hardware devices emulated
            by qemu.
 
-1af4:1110  ivshmem device (shared memory, docs/specs/ivshmem_device_spec.txt)
+1af4:1110  ivshmem device (shared memory, docs/specs/ivshmem-spec.txt)
 
 All other device IDs are reserved.