update exercise in 01_the_filesystem.md

[rkhetani]

Update the following exercise to have a cleaner output enabling students to check it against the instructor's output. Remove the Bonus question.

List all of the files in /bin that start with the letter 'c'
List all of the files in /bin that contain the letter 'a'
List all of the files in /bin that end with the letter 'o'

BONUS: List all of the files in /bin that contain the letter 'a' or 'c'.```