doc: ABI: vdso: update parse_vdso.c reference

Since commit f9b6b0ef ("selftests: move vDSO tests from Documentation/vDSO")
parse_vdso.c moved under selftests. Update the reference to match.

Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>