Remove return 0 (#44)

* Delete "return 0;" From do-not-fail.md

* Delete "return 0;" From lower-or-upper.md

Author: Fragiila

docs/text/chapter-4/practice/do-not-fail.md

@@ -70,6 +70,5 @@ int main() {
         }
     }
 
-    return 0;
 }
 ```

docs/text/chapter-4/practice/lower-or-upper.md

@@ -62,7 +62,6 @@ int main() {
     else {
         cout << "UPPERCASE" << endl;
     }
-    return 0;
 }
 ```
-:::
+:::