Disable playground on thread::sleep examples

The playground times out, and the point of these examples is seeing the
pause in between the printlns, which the playground doesn't do.

Fixes rust-lang#2607.

src/ch16-02-message-passing.md

@@ -3,7 +3,7 @@
 One increasingly popular approach to ensuring safe concurrency is *message
 passing*, where threads or actors communicate by sending each other messages
 containing data. Here’s the idea in a slogan from [the Go language
-documentation](https://golang.org/doc/effective_go.html#concurrency): 
+documentation](https://golang.org/doc/effective_go.html#concurrency):
 “Do not communicate by sharing memory; instead, share memory by communicating.”
 
 One major tool Rust has for accomplishing message-sending concurrency is the
@@ -176,7 +176,7 @@ pause for a second between each message.
 
 <span class="filename">Filename: src/main.rs</span>
 
-```rust
+```rust,noplayground
 {{#rustdoc_include ../listings/ch16-fearless-concurrency/listing-16-10/src/main.rs}}
 ```
 
@@ -219,7 +219,7 @@ so by cloning the transmitting half of the channel, as shown in Listing 16-11:
 
 <span class="filename">Filename: src/main.rs</span>
 
-```rust
+```rust,noplayground
 {{#rustdoc_include ../listings/ch16-fearless-concurrency/listing-16-11/src/main.rs:here}}
 ```