prereq/README.md: command to start mesheryctl should be `mesheryctl system start` instead of `mesheryctl start`

[daksh-sagar]

Running mesheryctl start throws Error: invalid command "start".

Can you please provide a better description for the issue?
Like where this change needs to happen?
Of course. In prereq/README.md file.

[Nikhil-Ladha]

Can you please provide a better description for the issue?
Like where this change needs to happen?

[leecalcote]

Good catch. Fixed.