pgsql: Replace defunct URL with stable archive.org URL in rbtree.c

Replace defunct URL with stable archive.org URL in rbtree.c

The URL for "Sorting and Searching Algorithms: A Cookbook"
by Thomas Niemann has started returning 404, and since we
refer to the page for license terms this replaces the now
defunct link with one to the copy on archive.org.

Author: Chao Li <lic@highgo.com>
Discussion: /messages/by-id/6DED3DEF-875E-4D1D-8F8F-7353D5AF7B79@gmail.com

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/6aa184c80f0cb0e20572441e0189567ed5176e1d

Modified Files
--------------
src/backend/lib/rbtree.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)