Fix documentation about size of interval type.

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

Author: tglsfdc

doc/src/sgml/datatype.sgml

@@ -1587,7 +1587,7 @@ SELECT E'\\xDEADBEEF';
        </row>
        <row>
         <entry><type>interval [ <replaceable>fields</replaceable> ] [ (<replaceable>p</replaceable>) ]</type></entry>
-        <entry>12 bytes</entry>
+        <entry>16 bytes</entry>
         <entry>time interval</entry>
         <entry>-178000000 years</entry>
         <entry>178000000 years</entry>