Sync docs and comments with code in hello_world example

The [hello_world.cpp](https://github.com/STEllAR-GROUP/hpx/blob/cfea04731607675f2bdea2bfc719af9d79a6a0af/examples/quickstart/hello_world.cpp) in [tutorial](http://stellar-group.github.io/hpx/docs/html/hpx/tutorial/examples/hello_world.html) uses `wait_all()` and `wait_each()`
functions, but the documentation and the comments state that `wait()`
function is used. This is not true since 50be59f8 and fca92db6.