Fixing exit prompt formatting

[ryanml]

Fixes: pkgxdev/ossapp#771

The consistent use of backticks matches what is present in the README (https://github.com/teaxyz/cli/blob/main/README.md?plain=1#L152)

[what-the-diff]

PR Summary

• Refinement to User Instruction
  The change made to the src/app.exec.ts file improves the user interaction. The modified instructions are clearer for the user when ending their session, now stating "when done type: exit". This adjustment enhances user experience and potentially reduces user errors.