Clean up E0593 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0593.md

@@ -11,3 +11,14 @@ fn main() {
     foo(|y| { });
 }
 ```
+
+You have to provide the same number of arguments as expected by the `Fn`-based
+type. So to fix the previous example, we need to remove the `y` argument:
+
+```
+fn foo<F: Fn()>(x: F) { }
+
+fn main() {
+    foo(|| { }); // ok!
+}
+```