Clarify that mac shorcuts should use Cmd instead of Ctrl

[carlosperate]

As using the Ctrl key could have other behaviour, and it might not be clear to all users that they should be using the Cmd key.

[prcutler]

Hi,

Here's the opening paragraph to the Shortcuts page today:

All the features in Mu can be accessed via keyboard shortcuts. Here’s how they work!

I would propose changing it to include macOS users:

All the features in Mu can be accessed via keyboard shortcuts. Windows and Linux
users should use the Control key as shown below.  macOS users should replace Control with Command.

Here's how they work!

Does that work or is there a preference to list the CMD shortcut next to each CTRL shortcut in the Shortcuts page?

I'm happy to submit a PR and am open to feedback.

Thanks!