races.md: data race -> race condition to violate memory safety

The first example shows that you "can't violate memory safety with safe
Rust" and the second example shows that you "can violate memory safety
with unsafe Rust".

The second example does not demonstrate a data race since there is only
one thread touching `data`.

Author: Enselic

src/races.md

@@ -60,8 +60,8 @@ thread::spawn(move || {
 println!("{}", data[idx.load(Ordering::SeqCst)]);
 ```
 
-We can cause a data race if we instead do the bound check in advance, and then
-unsafely access the data with an unchecked value:
+We can cause a race condition to violate memory safety if we instead do the bound
+check in advance, and then unsafely access the data with an unchecked value:
 
 ```rust,no_run
 use std::thread;