according to the idea of gtm-nayan #144, add println!("Success") for each exercise

src/variables.md

@@ -8,7 +8,9 @@
 fn main() {
     let x: i32; // uninitialized but using, ERROR !
     let y: i32; // uninitialized but also unusing, only warning
-    println!("{} is equal to 5", x); 
+
+    assert_eq!(x, 5);
+    println!("Success!")
 }
 ```
 
@@ -20,7 +22,8 @@ fn main() {
     let __ =  1;
     __ += 2; 
     
-    println!("{} is equal to 3", x); 
+    assert_eq!(x, 3);
+    println!("Success!")
 }
 ```
 
@@ -90,6 +93,8 @@ fn main() {
     let y = 4;
     // shadowing
     let y = "I can also be bound to text!"; 
+
+    println!("Success!")
 }
 ```
 
@@ -124,6 +129,8 @@ fn main() {
 
     assert_eq!(x, 3);
     assert_eq!(y, 2);
+
+    println!("Success!")
 }
 ```
 
@@ -142,6 +149,8 @@ fn main() {
     [.., y] = [1, 2];
     // fill the blank to make the code work
     assert_eq!([x,y], __);
+
+    println!("Success!")
 } 
 ```