fix apr_psprintf format string from e0df4e3dba7c4ab92442b9e82c1de01fdbaa...

...3cce

This resolves the issue first reported to APR in https://issues.apache.org/bugzilla/show_bug.cgi?id=56120.
(APR doesn't currently support the "z" prefix, though we are keeping that issue open as an enhancement request.)