[HttpClient] Update the concurrent requests section

[javiereguiluz]

Proposed changes:

• Update the intro to make it even more clear that this component is already "parallel by default", so you don't have to do anything special to send multiple requests
• Add the word parallel because it was not mentioned even once and "parallel requests" is a very common alternative term for "concurrent requests"
• Update the example because the Akamai URL is no longer active
• Some minor rewords to improve grammar and text flow