clarify that the map-reduce example relies on static data

[oconnor663]

The map-reduce example currently says "Because we're move-ing the data segments into the thread, Rust will also ensure the data is kept alive until the threads exit." Both halves of that sentence are true by themselves, but I think the "because" is misleading. The main reason Rust isn't giving us lifetime errors here is that the original data string is a &'static str constant. The move keyword is important for keeping the data static (by capturing &str instead of &&str), but it's not what made the data static. If you force the data to be non-static, like by adding .to_string() to the end of it, you can see that even with move this example doesn't compile:

error[E0597]: `data` does not live long enough
  --> src/main.rs:31:24
   |
31 |       let chunked_data = data.split_whitespace();
   |                          ^^^^^^^^^^^^^^^^^^^^^^^ borrowed value does not live long enough
...
55 |           children.push(thread::spawn(move || -> u32 {
   |  _______________________-
56 | |             // Calculate the intermediate sum of this segment:
57 | |             let result = data_segment
58 | |                 // iterate over the characters of our segment..
...  |
70 | |             result
71 | |         }));
   | |__________- argument requires that `data` is borrowed for `'static`
...
89 |   }
   |   - `data` dropped here while still borrowed

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r? @marioidival

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