pgsql: Fix documentation about size of interval type.

Fix documentation about size of interval type.

It's been 16 bytes, not 12, for ages.  This was fixed in passing in HEAD
(commit 146604ec), but as a factual error it should have been back-patched.
Per gripe from Tatsuhito Kasahara.

Branch
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REL9_0_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/640c0d20a17a0fcfc634b07319841727135b383a

Modified Files
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doc/src/sgml/datatype.sgml |    2 +-
1 file changed, 1 insertion(+), 1 deletion(-)