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>

Documentation/ABI/stable/vdso

@@ -16,7 +16,8 @@ The vDSO uses symbol versioning; whenever you request a symbol from the
 vDSO, specify the version you are expecting.
 
 Programs that dynamically link to glibc will use the vDSO automatically.
-Otherwise, you can use the reference parser in Documentation/vDSO/parse_vdso.c.
+Otherwise, you can use the reference parser in
+tools/testing/selftests/vDSO/parse_vdso.c.
 
 Unless otherwise noted, the set of symbols with any given version and the
 ABI of those symbols is considered stable.  It may vary across architectures,