Update Docstring (#1) (#451)

Updated the Docstring and corrected it for next_chunk()
diff --git a/googleapiclient/http.py b/googleapiclient/http.py
index f5d08a1..7992ea3 100644
--- a/googleapiclient/http.py
+++ b/googleapiclient/http.py
@@ -654,7 +654,7 @@
             request only once.
 
     Returns:
-      (status, done): (MediaDownloadStatus, boolean)
+      (status, done): (MediaDownloadProgress, boolean)
          The value of 'done' will be True when the media has been fully
          downloaded.