Rename StackOverflow on Learn JavaScript page

> Stack Overflow is a programmer Q&A site on the Stack Exchange Network.
> As a name, Stack Overflow, is always written "Stack Overflow" (two
> words, capital letters).

http://stackexchange.com/legal/trademark-guidance
Merge pull request #1730 from openjck/stack-space-overflow

Rename StackOverflow on Learn JavaScript page