Fix C++ deque demo and Deque/Dequeue title

[wrigjl]

In ImplementingaDequeCpp.ptx, the C++ deque demo had return 0; inside the final for loop, so after the pops only the first remaining item ("Turtle") was printed instead of the whole deque.

• Move the loop's closing brace above return 0;
• Add a trailing cout << endl; to match the first print loop
• Fix "Itm at the back" typo
• Retitle the exploration "Deque Implementation" (was "Dequeue", which is the queue operation, not the data structure)

Verified by extracting the program, compiling with -Wall (clean), and running it; the final line is now Turtle Zebra Catfish, matching the Python version.

... You can tell exactly where I am in the material this semester =)

[pearcej]

I can't beleive I missed this! Thanks!