Fix log section of sample config

[jsign]

Might be useful to compare tomlConfig struct and LogConfig struct. Steps to reproduce the problem (on fresh env, incl. transocks dependencies):

$ go get github.com/cybozu-go/transocks
$ cd cmd/transocks
$ cp sample.toml /etc/transocks.toml
$ go run main.go
2019-04-21T01:03:18.730787Z ihagopian main error: "undecoded key in TOML: [log_level log_file]"
exit status 1

Additionally from using table, the file field should be filename.

BTW, thanks for building transocks and writing the article.

[ymmt2005]

@jsign Thank you!