Update cpp-stl-cheatsheet.md

[prototype47]

Change Summary

Provide summary of changes with issue number if any.

Checklist

If you haven't fulfilled the below requirements or even delete the entire checklist, your PR won't be reviewed and will be closed without notice.

General

• [x] This Pull Request is all my own work. (You'll be blacklisted if you are caught for plagiarism.)
• [x] I've read CONTRIBUTING.md
• [x] I've made some valid changes to the CheatSheet, and they are not just minor changes.

Changes

• [x] I've read and followed the Solution Template

Note: To mark the checkbox, put an x inside the [ ] (like this: [x]).

[aryankashyap7]

@prototype47 These are some really small changes, So I cant issue the hacktoberfest tag over here.

But if you are interested in contributing to this specific cheat sheet, you can add the a tabular format of time and space complexity of various methods within the stl datastructures. That would be marked as a valid contribution.

eg:-

Vectors Methods and Time Complexity

Methods Name	Description	Time Complexity	Space Complexity
push_back()	Inserts an element at the end of the vector	O(1)	O(1)
pop_back()	Removes the last element of the vector	O(1)	O(1)