Typos, minor updates, or fixes to existing challenges

Whenever you are submitting any changes to the guide, please follow these recommendations:

• Always fork repository to your own account for applying modifications
• Do not combine multiple changes to one pull request, please submit any challenges and documentation updates using separate PRs
• If you are submitting multiple challenges, please create specific PR for each of them
• If you are submitting typo or documentation fix, you can combine modifications to single PR where suitable