pgsql: Fix ancient error in large objects usage example: overwrite()

Log Message:
-----------
Fix ancient error in large objects usage example: overwrite() subroutine
was opening with INV_READ flag and then writing.  Prior to 8.1 the backend
did not reject this, but now it does.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        lobj.sgml (r1.37 -> r1.38)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/lobj.sgml.diff?r1=1.37&r2=1.38)
    pgsql/src/test/examples:
        testlo.c (r1.25 -> r1.26)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/examples/testlo.c.diff?r1=1.25&r2=1.26)