Docs: remove instanceof from keyword-spacing docs

[mdjermanovic]

Prerequisites checklist

• I have read the contributing guidelines.

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

Prr the keyword-spacing docs, this rule enforces spacing around instanceof, but that isn't true. This rule does not enforce spacing around infix operators, that's the responsibility of space-infix-ops.

What changes did you make? (Give an overview)

Removed instanceof from the keyword-spacing docs. Also clarified that in applies to for-in, not the in operator.

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

[nzakas]

LGTM