Update index.md - Githubissues

Added more description to vectors

✅️ By submitting this PR, I have verified the following:

[x] Added descriptive name to PR
- Your PR should NOT be called Update index.md, since it does not help the maintainer understand what has been changed.
- For example, if you create a Variables article inside the Python directory, the pull request title should be Python: add Variables article.
- Additional PR title examples:
- Git: edit Git Commit article
- PHP: create PHP section and add Data Structures article
[x] Reviewed necessary formatting guidelines in CONTRIBUTING.md.
[x] No plagiarized, duplicate, or repetitive content that has been directly copied from another source.