[nit] fix comment of supervisor

[haya14busa]

The comment meant following code

if c == nil {
	c = make(chan os.Signal, 1)
	defer close(c)
}

but it actually looks like this

if c == nil {
	c = make(chan os.Signal, 1)
}
defer close(c)

I think both are fine but comment should be true.

ref: #327 (comment)

[Songmu]

🙇 Thanks!