docs: README: fix outdated git clone address (#3)

Author: lread

README.md

@@ -13,7 +13,7 @@ Here are example steps that can be followed to build Planck on for a given distr
 1. `cd ubuntu-14.04_64`
 2. `vagrant up`
 3. `vagrant ssh`
-4. `git clone https://github.com/mfikes/planck`
+4. `git clone https://github.com/planck-repl/planck.git`
 5. `cd planck`
 6. `script/build`