Fixed typo in documentation (#2733)

fs.go

@@ -17,7 +17,7 @@ type neuteredReaddirFile struct {
 	http.File
 }
 
-// Dir returns a http.Filesystem that can be used by http.FileServer(). It is used internally
+// Dir returns a http.FileSystem that can be used by http.FileServer(). It is used internally
 // in router.Static().
 // if listDirectory == true, then it works the same as http.Dir() otherwise it returns
 // a filesystem that prevents http.FileServer() to list the directory files.