Changes the default profile example to set prompt globally instead of locally

Problem:
The default example profile in posh-git provides a prompt function, as it does this in the current scope if you call this in a function the prompt on the shell is not modified.
I was attempting to create a function that would allow me to install posh-git only if I wanted to use it in a shell instead of in all shells when I hit this.

Proposed Solution:
Modify the prompt in the example to modify global:prompt instead of simply prompt - this means that it will work even if it is called in a lower scope.

I realise that this could just be modified by people that needed in the file they base on the example, but think it might be helpful to modify it here?