pgsql: Fix typo: < Such indexes could be more compact if there are

Log Message:
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Fix typo:

< Such indexes could be more compact if there are few unique value.

Such indexes could be more compact if there are only a few unique values.

Modified Files:
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pgsql/doc:
TODO (r1.1391 -> r1.1392)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/TODO.diff?r1=1.1391&amp;r2=1.1392)