Update index.yml

Author: dsyme

docs/fsharp/index.yml

@@ -56,7 +56,7 @@ landingContent:
           - text: "Functional programming concepts in F#"
             url: tutorials/functional-programming-concepts.md
           - text: "Using Functions in F#"
-            url: tutorials/functional-programming-concepts.md
+            url: tutorials/using-functions.md
           - text: "Async programming in F#"
             url: tutorials/async.md
           - text: "Type providers"