Update ThreadPoolDispatcher.kt

Fixed typos.
diff --git a/core/kotlinx-coroutines-core/src/main/kotlin/kotlinx/coroutines/experimental/ThreadPoolDispatcher.kt b/core/kotlinx-coroutines-core/src/main/kotlin/kotlinx/coroutines/experimental/ThreadPoolDispatcher.kt
index d9e1006..e877728 100644
--- a/core/kotlinx-coroutines-core/src/main/kotlin/kotlinx/coroutines/experimental/ThreadPoolDispatcher.kt
+++ b/core/kotlinx-coroutines-core/src/main/kotlin/kotlinx/coroutines/experimental/ThreadPoolDispatcher.kt
@@ -23,7 +23,7 @@
 import kotlin.coroutines.experimental.CoroutineContext
 
 /**
- * Creates new coroutine execution context with the a single thread and built-in [yield] and [delay] support.
+ * Creates a new coroutine execution context using a single thread with built-in [yield] and [delay] support.
  * **NOTE: The resulting [ThreadPoolDispatcher] owns native resources (its thread).
  * Resources are reclaimed by [ThreadPoolDispatcher.close].**
  *