Fixed indentation error.

Hi, Mathieu! I like QTodoTxt a lot. I discovered that one line in app.py starts with a tab; my change replaces the tab with spaces to match all other lines. It might also be worth thinking about the reason I discovered this: the main executable starts with

    #!/usr/bin/env python

On my system (Arch Linux), this invokes python3, and in python3 the inconsistent indentation is an error. And I see that the app won't work with python3, so for myself I need to change the header to

    #!/usr/bin/env python2

I didn't commit that change because I'm not sure that is the right thing to do for all users. But it might be a good thing to consider.

Thanks!