Fix item kind

[kitagry]

before

p(arg) { } // function

p[arg] { } // function

p = arg { } // function

after

p(arg) { } // function

p[arg] { } // function

p = arg { } // variable

[kitagry]

I think the rule having variable is a kind of function. But, I'm not confident...

I read documents more carefully, and then decide it.

[kitagry]

Variables appearing in the head of a rule can be thought of as input and output of the rule.

So, it can be thought as function. I will merge this PR.