Small problems in double-ended queues documentation

Created by: stephanemagnenat

There are the following problems in the double-ended queues documentation:

• The subtitle should be "Double-ended queues" or "Container: double-ended queue", not "Native functions for double-ended queues"
• In the first sentence, the link to Wikipedia is spelled out by mistake.
• The first entry, deque.size(Q,n), is not properly indented.