concurrency: avoid 1.. loops (#2280)

Fixes #2060. Note that this does not change the "blocking executor"
example because on that slide it is worthwhile to sleep for 1 * 10ms on
the first iteration and so on. But we shouldn't use one-indexed
inclusive loops when the only significant feature of the loop is its
iteration count.

src/concurrency/shared-state/arc.md

@@ -13,7 +13,7 @@ use std::thread;
 fn main() {
     let v = Arc::new(vec![10, 20, 30]);
     let mut handles = Vec::new();
-    for _ in 1..5 {
+    for _ in 0..5 {
         let v = Arc::clone(&v);
         handles.push(thread::spawn(move || {
             let thread_id = thread::current().id();