Fix sample code for ReactorContext (Kotlin#2682)

* Also add toString implementation

Author: dkhalanskyjb

reactive/kotlinx-coroutines-reactor/api/kotlinx-coroutines-reactor.api

@@ -35,6 +35,7 @@ public final class kotlinx/coroutines/reactor/ReactorContext : kotlin/coroutines
 	public fun <init> (Lreactor/util/context/Context;)V
 	public fun <init> (Lreactor/util/context/ContextView;)V
 	public final fun getContext ()Lreactor/util/context/Context;
+	public fun toString ()Ljava/lang/String;
 }
 
 public final class kotlinx/coroutines/reactor/ReactorContext$Key : kotlin/coroutines/CoroutineContext$Key {

reactive/kotlinx-coroutines-reactor/src/ReactorContext.kt

@@ -27,18 +27,18 @@ import reactor.util.context.*
  * ```
  * val flux = myDatabaseService.getUsers()
  *     .contextWrite { ctx -> println(ctx); ctx }
- * flux.await() // Will print "null"
+ * flux.awaitFirst() // Will print "null"
  *
  * // Now add ReactorContext
  * withContext(Context.of("answer", "42").asCoroutineContext()) {
- *    flux.await() // Will print "Context{'key'='value'}"
+ *    flux.awaitFirst() // Will print "Context{'key'='value'}"
  * }
  * ```
  *
  * #### Propagating subscriber's Context to ReactorContext:
  * ```
  * val flow = flow {
- *     println("Reactor context in Flow: " + coroutineContext[ReactorContext])
+ *     println("Reactor context in Flow: " + currentCoroutineContext()[ReactorContext])
  * }
  * // No context
  * flow.asFlux()
@@ -55,6 +55,8 @@ public class ReactorContext(public val context: Context) : AbstractCoroutineCont
     public constructor(contextView: ContextView): this(Context.of(contextView))
 
     public companion object Key : CoroutineContext.Key<ReactorContext>
+
+    override fun toString(): String = context.toString()
 }
 
 /**