Docs: fix a misleading example in one-var

[aladdin-add]

What is the purpose of this pull request? (put an "X" next to item)

[ x] Documentation update
[ ] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofixing to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

What changes did you make? (Give an overview)
splitting the code to 2 blocks for less confusion.

Is there anything you'd like reviewers to focus on?

should we do the same thing to other code blocks?
e.g.
Examples of incorrect code for this rule with the default "always" option:

/*eslint one-var: ["error", "always"]*/
/*eslint-env es6*/

function foo() {
    var bar;
    var baz;
    let qux;
    let norf;
}

function foo(){
    const bar = false;
    const baz = true;
    let qux;
    let norf;
}

function foo() {
    var bar;

    if (baz) {
        var qux = true;
    }
}

the code is valid, but redeclaring the vars isn't a good practice.

[aladdin-add]

not sure why the tests not passed -- not related to this change.

[g-plane]

It's strange that the CI failed while you haven't changed the code.

What about running locally?

[aladdin-add]

seems it comes from acorn@6.0.7, it fixed some bugs that some tests relying on.

[g-plane]

LGTM, thanks!

[not-an-aardvark]

LGTM, thanks!