Clarify that tuples are fixed in size?

[nirbheek]

In Chapter 3, data types, the section on arrays explicitly says that arrays are fixed size, but the section on tuples does not, so I was confused whether tuples are fixed sized or not in Rust.

https://github.com/rust-lang/book/blob/master/2018-edition/src/ch03-02-data-types.md#the-tuple-type