README: fix the git clone to use a syntax that works

The former style is deprecated

Author: bagder

README.md

@@ -10,7 +10,7 @@ A command line tool that reads nroff and outputs HTML
 
 ## Code
 
- git clone git://github.com/bagder/roffit.git
+ git clone https://github.com/bagder/roffit.git
 
 ## Contributors