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

see kodecocodes#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.

Author: ColinEberhardt

README.markdown

@@ -15,7 +15,7 @@ Writing Objective-C? Check out our [Objective-C Style Guide](https://github.com/
 * [Classes and Structures](#classes-and-structures)
   * [Use of Self](#use-of-self)
 * [Function Declarations](#function-declarations)
-* [Closures](#closures)
+* [Closure Expressions](#closure-expressions)
 * [Types](#types)
   * [Constants](#constants)
   * [Optionals](#optionals)
@@ -193,7 +193,7 @@ The example above demonstrates the following style guidelines:
 
 For conciseness, avoid using `self` since Swift does not require it to access an object's properties or invoke its methods.
 
-Use `self` when required to differentiate between property names and arguments in initializers, and when referencing properties in closures to make capture semantics explicit:
+Use `self` when required to differentiate between property names and arguments in initializers, and when referencing properties in closure expressions (as required by the compiler):
 
 ```swift
 class BoardLocation {
@@ -230,7 +230,7 @@ func reticulateSplines(spline: [Double], adjustmentFactor: Double,
 ```
 
 
-## Closures
+## Closure Expressions
 
 Use trailing closure syntax wherever possible. In all cases, give the closure parameters descriptive names: