Improved readability of second paragraph

Resolved grammatical issues and more thoroughly introduced the map
function.
Fixed grammar and typo

Switched an “it’s” to “its” and added a missing quote.
Improved readability of HOF transition

Improved the readability of the transition from two simple functions to
HOF functions.
Merge pull request #39 from hasselg/ho_functions

Corrected grammar and improved clarity of HOF descriptions