Update SingleNumber Implementation to Use Bitwise XOR #143

[harshendram]

I propose updating the implementation to use the XOR bitwise operation, which provides a more efficient solution. The XOR approach has a time complexity of O(n) but reduces the space complexity to O(1), as it does not require any additional data structures.

• [x] I have read CONTRIBUTING.md.
• [x] This pull request is all my own work -- I have not plagiarized it.
• [x] All functions and variable names follow Clean Code naming conventions.
• [ ] This code follows the clang-format style.