pgsql: Adjust documentation wording of window function ORDER BY to not

Adjust documentation wording of window function ORDER BY to not mention
avg() because it was inaccurate.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/d5813488a4ccc78ec3a4ad0d5da4e6e844af75e8

Modified Files
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doc/src/sgml/advanced.sgml | 8 ++++----
1 files changed, 4 insertions(+), 4 deletions(-)