Changed "closure" to "closure expression" in a few locations

see #35

Note, Closure Expressions create Closures, hence the two terms can be used interchangeably in many instances. Apple does this in their language reference, e.g. they use "trailing closure syntax" rather than "trailing closure expression syntax".

However, there are a few instances in our style guide where I think 'Closure Expression' makes more sense.
Merge pull request #67 from ColinEberhardt/master

Changed "closure" to "closure expression" in a few locations