Fix pod syntax in the README

Author: ricsantos

README.markdown

@@ -8,7 +8,7 @@ This project supports Swift 3.0, 3.1, 3.2 and 4.0; iOS 9, 10 and 11; tvOS 10 and
 ## CocoaPods
 
 ```ruby
-pod "PromiseKit/UIKit" ~> 4.0
+pod "PromiseKit/UIKit", "~> 4.0"
 ```
 
 The extensions are built into `PromiseKit.framework` thus `import PromiseKit` is