Update README.md (#12)

Fix example usage to show arguments

Author: leventkantaroglu

README.md

@@ -4,6 +4,6 @@ To build this example package:
 
     git clone https://github.com/apple/example-package-dealer.git
     cd example-package-dealer
-    swift run
+    swift run dealer <count>
 
 For more information, visit [Swift's package manager documentation](https://www.swift.org/package-manager/).