Example does not make it clear whether one can use short-lived references

The example uses `&1`, which (due to promotion) is a `&'static` reference. So the example code could easily be written with regular `std` `thread_local!`s. `static FOO: u32` also makes it look like the thread-local stores a value, not a reference.

I think the entire point of this crate is to allow thread-locals storing short-lived references, but the example doesn't really make that clear.