docs(contributing): fix incorrect vector iteration and simplify quick_sort example

[Aditya-354]

Description of Change

This PR fixes and improves two example snippets in the "Self-test examples" section of CONTRIBUTING.md:

1. Improved quick_sort example

   • Replaced the overly complex expression
     int(std::end(arr) - std::begin(arr)) - 1
     with the clearer and equivalent arr.size() - 1.
   • This improves readability and makes the example easier for beginners to understand.

2. Fixed and optimized is_number_on_array example

   • The original loop used sizeof(arr) / sizeof(int) which is incorrect for vectors and results in invalid iteration.
   • Replaced it with a safer and correct loop using arr.size().
   • Further improved it by converting the loop to a modern range-based for-loop:

     for (const auto &x : arr) {
         if (x == number) return true;
     }

   • This makes the example more idiomatic, readable, and efficient.

These changes enhance correctness, clarity, make it more beginner-friendly, and implement modern C++ best practices in the contribution examples.

Checklist

• Added description of change
• Updated example documentation/code
• PR title follows semantic commit guidelines
• I acknowledge that all my contributions will be made under the project's license.