Improved description for task 9.

Author: javadev

src/main/java/g0001_0100/s0009_palindrome_number/readme.md

@@ -36,6 +36,6 @@ An integer is a **palindrome** when it reads the same backward as forward. For e
 
 **Constraints:**
 
-*   `-231 <= x <= 231 - 1`
+*   <code>-2<sup>31</sup> <= x <= 2<sup>31</sup> - 1</code>
 
 **Follow up:** Could you solve it without converting the integer to a string?