Update cours_12.md

slides/cours_12.md

@@ -108,11 +108,10 @@ void queue_enqueue(queue *fa, int val) {
     elmt->next = NULL;
     if (queue_is_empty(*fa)) {
         fa->head = elmt;
-        fa->tail = elmt;
     } else {
-        fa->tail->next = elmt;
-        fa->tail = elmt;
+        fa->tail->next = elmt;  
     }
+    fa->tail = elmt;
 }
 ```