anything that is a `Number` is <: Number (not :>)

1. Anything that is a `Number` is `<: Number` and not `:> Number`
2. Clarification about Only a `Number`, clearer to say its Only that items which are of a type of a Number.
Merge pull request #25 from tomer-ben-david-forked/gh-pages

anything that is a `Number` is `<: Number` (not `:>`)