docs/bisect-lk2009: update nist report link
The original NIST press release linked here is no longer
available. But it was just a one-page summary of a larger
planning report; we can link to the report and point people
to the executive summary, which contains the same
information.
Ideally we'd cite it with a DOI, but I couldn't dig one up
for this particular document. I found many URLs pointing to
this report, but they all end up redirecting to this one
(and it looks somewhat official).
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>