README: Update Swift version badge to 5.5

Author: JohnSundell

README.md

@@ -3,7 +3,7 @@
 </p>
 
 <p align="center">
-    <img src="https://img.shields.io/badge/Swift-5.4-orange.svg" />
+    <img src="https://img.shields.io/badge/Swift-5.5-orange.svg" />
     <a href="https://swift.org/package-manager">
         <img src="https://img.shields.io/badge/swiftpm-compatible-brightgreen.svg?style=flat" alt="Swift Package Manager" />
     </a>