pgsql: Update doc mention of diskchecker.pl to add URL for script; reta

Update doc mention of diskchecker.pl to add URL for script; retain URL
for description.

Patch to 9.0 and later, where script is mentioned.

Branch
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REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/3d980e15eeeecc4cc240c45dc5abb5567adce4bb

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