Little typo fix

- ./hello_world which is not recognized on Windows fixed.
- .exe extension added.
- Little rewriting.
auto merge of #15247 : smenardpw/rust/patch-1, r=alexcrichton

./hello_world is not recognized on Windows.
We can type either hello_world or hello_world.exe to run the executable. I chose "hello_world.exe", which seems more conventional on Windows.