Hi Alfred,
I’m afraid that currently there’s no option to change it from “Yes” to “✓”. However, as a workaround, you can use image for that purpose (meaning a “✓” image instead of using a boolean field). If you wish to sort / query the collection based on this field, you can have two columns (the first one is image column and the other one is boolean column) and hide the boolean column from the table.
Good luck!
Tal.