Typos, grammatical errors fix and improve player check in join

[FarzeenKist]

Changes

1. Typos fixes
2. Grammatical errors fixes
3. Added title for tutorial
4. Optimized the check for whether the caller is already a player by adding a mapping named entered and modifying the if statement. This fixes the scalability issues and makes the function execution time relatively constant instead of depending on the players' array size.