Update parseHelper in F# tour to account for span overloads (#18689)

Fixes #18609

Author: cartermp

samples/snippets/fsharp/tour.fs

@@ -745,7 +745,7 @@ module PatternMatching =
 
     /// You can also use the shorthand function construct for pattern matching, 
     /// which is useful when you're writing functions which make use of Partial Application.
-    let private parseHelper f = f >> function
+    let private parseHelper (f: string -> bool * 'T) = f >> function
         | (true, item) -> Some item
         | (false, _) -> None