Fix bug causing bot to reply to its own messages in event_handler example

[Spacerulerwill]

In the event_handler example, sending message containing the word "poise" would cause the bot to reply with the message "Poise has been mentioned {n} times" like expected. However, since the bot's message itself contained the word "poise", it would continually reply to itself.

I fixed it by adding a simple check to test the message_author's id against the bot's user id.

[kangalio]

Thank you! The CI is still failing, but then we can merge

[Spacerulerwill]

I have formatted the file properly with rustfmt and have ammended my commit