doc/interface: correct signature of size

[iblislin]

I think size(A, [dim...]) has been deprecated.

julia> size(A, 1,2)
┌ Warning: `size(x, d1::Integer, d2::Integer)` is deprecated, use `(size(x, d1), size(x, d2))` instead.
│   caller = top-level scope at none:0
└ @ Core none:0