minor text changes

This pull request makes minor changes to the text. I can revise the request if required.

When I worked through this tutorial, I didn't realise that async.each works like Array.forEach, until I looked at the async documentation. I've changed the text to make this clearer. If you think other students will understand this more quickly than me, then not all the changes may be necessary.

1) Lines 1 to 3 - I've removed "series" and made the wording clearer that the same function is used repeatedly. Added "to" before "check". Changed "do three" to "make three calls using the values in the array". Added a specific mention of async.each. Minor tidying of newlines.

2) Line 31 to 33 - Minor tidying of newline. Comma after 'http.get'. Added "and" before "console.log".