README: Launch ui5 serve without slash in open param

Using slashes in commands might lead to issues when using the Git Bash on Windows

Author: RandomByte

README.md

@@ -20,7 +20,7 @@
 
 1. Start a local server and run the application (http://localhost:8080/index.html)
     ```sh
-    ui5 serve -o /index.html
+    ui5 serve -o index.html
     ```
 
 ## Testing