This kind of advice confused me early on because I didn't realize the : was part of the command for way too long. For beginners, you might want to spell it out further as: "shift and the : key, then qa, then Enter"
You're only targetting old 1970s terminal users, there, though.
If you wanted to also help slightly more modern users who are used to CUA conventions from GUIs, you would have the [F1] key bring up some form of help screen, which said something like "Get out of Vim: Use :qa!" at the top.
You're only targeting old 1990s GUI users there, though.
If you wanted to also help silghtly more modern users who are used to discoverable-UI conventions from mobile apps, you would have it so shaking your device brings up some form of chat bot, which would suggest asking it how to quit Vim.
Something along the lines of "Type :qa and press <Enter> to exit Vim" would probably do it...