Small Fix && cargo fmt

user/src/bin/stack_overflow.rs

@@ -5,7 +5,7 @@
 extern crate user_lib;
 
 fn f(d: usize) {
-    println!("d = {}",d);
+    println!("d = {}", d);
     f(d + 1);
 }
 
@@ -14,4 +14,4 @@ pub fn main() -> i32 {
     println!("It should trigger segmentation fault!");
     f(0);
     0
-}
+}