Typo: and -> an

Author: Andrew Ash <andrew@andrewash.com>

Closes #927 from ash211/patch-5 and squashes the following commits:

79b577d [Andrew Ash] Typo: and -> an

(cherry picked from commit 9c1f204d807403a7cb4099c29077e1f87f466179)
Signed-off-by: Reynold Xin <rxin@apache.org>