Change wording of the Terminal welcome message on macOS

[michalrus]

Follow-up to:

• Create a DMG release artifact for macOS #146

Requested by @opacular in this Slack thread.

Current state

The current form is:

  Hello!

  This terminal has been set up to allow easy running of 'blockfrost-platform'.

  Try typing in one of:

    blockfrost-platform --help
    blockfrost-platform --init

  … and pressing <ENTER>.

You can use colors, but anything outside of the basic 16 terminal colors is risky, and we can't control how they are configured to look on the end machine. So we've only used bold so far.