chore(docs): rake secret was deprecated, do rails secret

[conduct0]

Summary

Minor fix of the README. When running recommended command to create a secret I ran into an error.

❯ bundle exec rake secret           
rake aborted!
Don't know how to build task 'secret' (See the list of available tasks with `rake --tasks`)
/home/thaymonkey/.rbenv/versions/3.1.2/bin/bundle:25:in `load'
/home/thaymonkey/.rbenv/versions/3.1.2/bin/bundle:25:in `<main>'
(See full trace by running task with --trace)

Seems like in the last rails versions secret was moved from rake to rails. See this post in Stack Overflow for more info https://stackoverflow.com/questions/77855137/dont-know-how-to-build-task-secret

Checklist

The following are mandatory for all PRs:

• I have written a thorough PR description.
• I have kept my commits small and atomic.
• I have used clear, explanatory commit messages.

The following are not always needed (cross them out if they are not):

• I have added automated tests to cover my changes.
• I have attached screenshots to demo visual changes.
• I have opened a PR to update the guides.
• I have updated the README to account for my changes.

[waiting-for-dev]

Thanks!