When you want to use a variable in a function, you must declare it first. Plus you don't always need to use a variable when you are calling a function. Just write "0" instead of "page" or whatever you want.
Besides, If you don't define a page number in menu_display(), it will show the first page automatically. If you look at syntax of the function, you will see
menu_display( index, menu, page=0 )
page=0 means, if you don't type a page, it will be 0 by default.