A DESCRIPTION OF THE PROBLEM :
Typo.
Should be "indicate", not "indicated".

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
This is so because null is used as a special return value by various methods to indicate that the deque is empty.
ACTUAL -
This is so because null is used as a special return value by various methods to indicated that the deque is empty.

URL OF FAULTY DOCUMENTATION :
http://docs.oracle.com/javase/8/docs/api/