Short cut for switching between opened documents executes zoom out

Hi to all,

I open two documents in Omnigraffle on macOS. Now I want to switch between them by using the shortcut ‘cmd <>’ (German keyboard what should be equally to ‘cmd ~’ in the US keyboard), but it does not switch between the opened documents. Instead, it zooms out.

Can someone tell me the correct shortcut or tell me how I can assign this function to the shortcut?

Many thanks in advance.