doc: incorrect description about log_dir

[yaojingguo]

https://github.com/google/glog/blob/master/README.rst#severity-levels says:

Unless otherwise specified, glog writes to the filename /tmp/<program name>.<hostname>.<user name>.log.<severity level>.<date>-<time>.<pid> (e.g., /tmp/hello_world.example.com.hamaji.log.INFO.20080709-222411.10474).

The truth is that if TMPDIR is set, it will be used instead of /tmp.