Merge pull request #243 from AananthV/patch-2

Update concurrency.md

Author: nrc

src/part-guide/concurrency.md

@@ -22,7 +22,7 @@ do_another_thing();
 
 Each statement is completed before the next one starts[^obs1]. Nothing happens in between those statements[^obs2]. This might sound trivial but it is a really useful property for reasoning about our code. However, it also means we waste a lot of time. In the above example, while we're waiting for `println!("hello!")` to happen, we could have executed `do_another_thing()`. Perhaps we could even have executed all three statements at the same time.
 
-Even where IO[^io-def] happens (printing using `println!` is IO - it is outputting text to the console via a call to the OS), the program will wait for the IO to complete[^io-complete] before executing the next statement. Waiting for IO to complete before continuing with execution *blocks* the program from making other progress. Blocking IO is the easiest kind of IO to use, implement, and reason about, but it is also the least efficient - in a sequential world, the program can do nothing while it waits for the IO to complete.
+Whenever IO[^io-def] happens (printing using `println!` is IO - it is outputting text to the console via a call to the OS), the program will wait for the IO to complete[^io-complete] before executing the next statement. Waiting for IO to complete before continuing with execution *blocks* the program from making other progress. Blocking IO is the easiest kind of IO to use, implement, and reason about, but it is also the least efficient - in a sequential world, the program can do nothing while it waits for the IO to complete.
 
 [^obs1]: This isn't really true: modern compilers and CPUs will reorganize your code and run it any order they like. Sequential statements are likely to overlap in many different ways. However, this should never be *observable* to the program itself or its users.
 [^obs2]: This isn't true either: even when one program is purely sequential, other programs might be running at the same time; more on this in the next section.