Update tut. to not sound like I missed a section

The sentence "Remember that `(float, float)` is a tuple of two floats"
sounds like you've already read a section on tuples, but that section
comes later. Changing it to "Assuming that ..." makes it more about
taking the writer's word that the syntax is how tuples are defined.
auto merge of #5707 : brettcannon/rust/patch-1, r=thestinger

The sentence "Remember that `(float, float)` is a tuple of two floats"
sounds like you've already read a section on tuples, but that section
comes later. Changing it to "Assuming that ..." makes it more about
taking the writer's word that the syntax is how tuples are defined.