pgsql: Fix incorrect markup in documentation of window frame clauses.

Fix incorrect markup in documentation of window frame clauses.

You're required to write either RANGE or ROWS to start a frame clause,
but the documentation incorrectly implied this is optional.  Noted by
David Johnston.

Branch
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REL9_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/68a8f26e91a15294024270eb3eed4468dbb307cf

Modified Files
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doc/src/sgml/ref/select.sgml |    4 ++--
doc/src/sgml/syntax.sgml     |    4 ++--
2 files changed, 4 insertions(+), 4 deletions(-)