Fix the "Python version" documentation

* repo needs python 2.6 - 2.7, any other version
  doesn't work. [1]

[1]: https://gerrit.googlesource.com/git-repo/+/51813dfed1528307c46707bc38b4db33b87bb36e

Change-Id: Id13cca892a00ab992fa3e9b3cbc5a824ede7e816
diff --git a/src/source/building.md b/src/source/building.md
index e2de012..dc1388a 100644
--- a/src/source/building.md
+++ b/src/source/building.md
@@ -146,7 +146,7 @@
 
 ## Python Version 3 ##
 
-Repo is built on particular functionality from Python 2.x and is unfortunately incompatible with Python 3.  In order to use repo, please install Python 2.x:
+Repo is built on particular functionality from Python 2 and is unfortunately incompatible with Python 3.  In order to use repo, please install Python 2.6 - 2.7:
 
     $ apt-get install python