fixed README demo code symbol error

README.md

@@ -296,7 +296,7 @@ Adding a custom HTTP header to a `Request` is supported directly in the global `
 
 ```swift
 let headers = [
-    "Authorization", "Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==",
+    "Authorization": "Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==",
     "Content-Type": "application/x-www-form-urlencoded"
 ]