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.

Tags:
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REL8_1_STABLE

Modified Files:
--------------
    pgsql/doc/src/sgml:
        lobj.sgml (r1.37 -> r1.37.2.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/lobj.sgml.diff?r1=1.37&r2=1.37.2.1)
    pgsql/src/test/examples:
        testlo.c (r1.25 -> r1.25.6.1)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/examples/testlo.c.diff?r1=1.25&r2=1.25.6.1)