pgsql: Doc: shorten JSON_SERIALIZE example to avoid a PDF build warning

Doc: shorten JSON_SERIALIZE example to avoid a PDF build warning.

There's no particular reason why this example has to use a
3-element array rather than 2-element.  Shortening it makes
the result bytea narrow enough to not cause a margin overrun
in A4 format.

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/7b735f8b52ad4ccf742c29dc41e3d049bdffe2fa

Modified Files
--------------
doc/src/sgml/func.sgml | 9 ++++-----
1 file changed, 4 insertions(+), 5 deletions(-)