update badge for swift

Author: RockfordWei

README.md

@@ -23,10 +23,7 @@
 
 <p align="center">
     <a href="https://developer.apple.com/swift/" target="_blank">
-        <img src="https://img.shields.io/badge/Swift-3.1-orange.svg?style=flat" alt="Swift 3.1">
-    </a>
-    <a href="https://developer.apple.com/swift/" target="_blank">
-        <img src="https://img.shields.io/badge/Swift-4.0-orange.svg?style=flat" alt="Swift 4.0">
+        <img src="https://img.shields.io/badge/Swift-5.2-orange.svg?style=flat" alt="Swift 5.2">
     </a>
     <a href="https://developer.apple.com/swift/" target="_blank">
         <img src="https://img.shields.io/badge/Platforms-OS%20X%20%7C%20Linux%20-lightgray.svg?style=flat" alt="Platforms OS X | Linux">