Issue#8769 - Fix oid search extension syntax in documentation

The option string, e.g. '=:dn:' is parsed like a LDIF entry starting from the '=' and replacing the '=' with a dummy variable 'x'. In this case, said string is 'x:dn:', so the resulting effective value is 'dn:'. This also implies that base64 values have to be passed in the form '=::'.