Simplify Lesson 11.

Author: isaacabraham

src/code-listings/lesson-11.fsx

@@ -23,7 +23,7 @@ let buildDtThisMonth = buildDtThisYear DateTime.UtcNow.Month
 // Listing 11.5
 open System.IO
 let writeToFile (date:DateTime) filename text =
-    let path = sprintf "%O-%s.txt" (date.ToString "yyMMdd") filename
+    let path = sprintf "%s-%s.txt" (date.ToString "yyMMdd") filename
     File.WriteAllText(path, text)
 
 // Listing 11.6