pgsql: Now that the diskchecker.pl author has updated the download link

Now that the diskchecker.pl author has updated the download link on his
website, revert the separate link to the download git repository.

Backpatch from 9.0 to current.

Branch
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REL9_0_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/85509b9c646e2598b3d664b4d7afc16ea41ea708

Modified Files
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doc/src/sgml/wal.sgml | 6 ++----
1 files changed, 2 insertions(+), 4 deletions(-)