Fix code sample for log()

Run on java9(9.0.4)
The log(msg: String) doesn't work like the description in document.

`fun log(msg: String) = println("[${Thread.currentThread().name}] $msg")`

The output is not same like expected in document:

```
[main] Unconfined
[ForkJoinPool.commonPool-worker-2] Default
[main] main runBlocking
[MyOwnThread] newSingleThreadContext
[kotlinx.coroutines.DefaultExecutor] Unconfined
```
I change to:

`fun log(msg: String) = Thread.currentThread().run { println("[$name @coroutine#$id] $msg") }`

output:

```
[main @coroutineKotlin#1] Unconfined
[ForkJoinPool.commonPool-worker-2 @coroutineKotlin#15] Default
[main @coroutineKotlin#1] main runBlocking
[MyOwnThread @coroutineKotlin#17] newSingleThreadContext
[kotlinx.coroutines.DefaultExecutor @coroutineKotlin#13] Unconfined
```

Author: XinyueZ

coroutines-guide.md

@@ -1129,7 +1129,7 @@ import kotlin.coroutines.experimental.*
 -->
 
 ```kotlin
-fun log(msg: String) = println("[${Thread.currentThread().name}] $msg")
+fun log(msg: String) = Thread.currentThread().run { println("[$name @coroutine#$id] $msg") }
 
 fun main(args: Array<String>) = runBlocking<Unit> {
     val a = async {