* Update to index.md + code fix
Before this commit, the js and c++ versions of the code had the O(n) modification with the flag but the Java, Python, Swift ones did not. Also, Bubble sort is originally O(n^2) for all cases and the flag is a modification to it, as far as I know. I updated the code to reflect the same in the first section, and added a new section explaining the modification with examples in 3 languages. Hope this is alright.
* fix: corrected code block syntax
0213ee1a13