The shortcut the Apple menu reports for the Force Quit menu item is ⌥⌘⎋. (the last character is the one Unicode calls broken circle with northwest arrow.)
What key should I press to invoke the Force Quit menu item?
The Escape key.
I don't know why Apple use this non-standard symbol, but they always have...
Here is a full list of all of the symbols that Apple use.
As an extension to Ian's answer, here's a list of all the keys:
As correctly pointed out by Ian, the ⎋ is the Escape key located on the upper left side of your keyboard.
You can find a list of all the keyboard icons, terminology, and HTML entities over here on Meta . That icon ⎋ is the Escape key. You can find it in the very top left-hand corner of your keyboard.