Update chapter_3_4.md

src/chapter_3/chapter_3_4.md

@@ -68,12 +68,12 @@ pub fn add_one(x: i32) -> i32 {
 /** `add_two` 将指定值加2
 
 
-```
+\```
 let arg = 5;
 let answer = my_crate::add_two(arg);
 
 assert_eq!(7, answer);
-```
+\```
 */
 pub fn add_two(x: i32) -> i32 {
     x + 2