Tin: SelectMarkGrpUnread is not invocable by 'Z'

Hello, all.

The man page for tin 2.6.5 release 20251224 says:

    SelectMarkGrpUnread 'z' 'Z'

yet in practice only 'z' invokes this command, whereas 'Z' ends
with:

	 Bad command. Type 'h' for help.

I think it is an error, either in Tin or in its documentation.