Update max function in tutorial ;; update libstd docs for slpop and slpush

[henesy]

Change max function in tutorial to compile.

Update libstd slice manipulation docs for slpop and slpush.

says:

generic slpush : (sl : @a[:], elt : @a -> @a[:])

is:

generic slpush : (sl : @a[:]#, elt : @a -> @a[:])

says:

generic slpop : (sl : @a[:] -> (@a, @a[:]))

is:

generic slpop : (sl : @a[:]# -> @a)