Update Purity code examples to use interpolation

Also, add in the missing argument to `greet` in the changing-global-state example code.

Author: alexlafroscia

readme.md

@@ -179,7 +179,7 @@ A function is pure if the return value is only determined by its
 input values, and does not produce side effects.
 
 ```js
-const greet = (name) => 'Hi, ' + name
+const greet = (name) => `Hi, ${name}`
 
 greet('Brianne') // 'Hi, Brianne'
 ```
@@ -189,9 +189,7 @@ As opposed to each of the following:
 ```js
 window.name = 'Brianne'
 
-const greet = () => {
-  return 'Hi, ' + window.name
-}
+const greet = () => `Hi, ${window.name}`
 
 greet() // "Hi, Brianne"
 ```
@@ -202,10 +200,10 @@ The above example's output is based on data stored outside of the function...
 let greeting
 
 const greet = (name) => {
-  greeting = 'Hi, ' + name
+  greeting = `Hi, ${name}`
 }
 
-greet()
+greet('Brianne')
 greeting // "Hi, Brianne"
 ```