change tabs to spaces

As I am about to submit some additions to CiviRules, I wanted to explore what the civirules style is regarding indentation. I discovered that two spaces (four in some places in docs/*.md) and no tabs are normally used. So I cleaned the few places where tabs had crept in. This MR is the result. I would normally lean towards tabs but I guess it is in your interest to merge this :-) Also I will of course submit my new code in the same style.