Fix rustdoc errors

Author: GuillaumeGomez

src/hello/print/print_display.md

@@ -110,7 +110,8 @@ each requires its own implementation. This is detailed further in
 After checking the output of the above example, use the `Point2D` struct as
 guide to add a Complex struct to the example. When printed in the same
 way, the output should be:
-```
+
+```txt
 Display: 3.3 +7.2i
 Debug: Complex { real: 3.3, imag: 7.2 }
 ```

src/macros/variadics.md

@@ -33,7 +33,8 @@ fn main() {
 ```
 
 Output:
-```
+
+```txt
 1 + 2 = 3
 3 + 4 = 7
 (2 * 3) + 1 = 7