[PR #1927] Fixed bug in README example code around default headers.

The example code in the "Modifying the Session Configuration" section was calling the `defaultHTTPHeaders` property as an instance property rather than static.

README.md

@@ -913,7 +913,7 @@ let sessionManager = Alamofire.SessionManager(configuration: configuration)
 #### Modifying the Session Configuration
 
 ```swift
-var defaultHeaders = Alamofire.SessionManager.default.defaultHTTPHeaders
+var defaultHeaders = Alamofire.SessionManager.defaultHTTPHeaders
 defaultHeaders["DNT"] = "1 (Do Not Track Enabled)"
 
 let configuration = URLSessionConfiguration.default