website: Added missing python-six dependency for Ubuntu 18.04

Change-Id: I32e0a5606149568dcf592fbe2200262ed413c47e
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/28871
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: Jason Lowe-Power <power.jg@gmail.com>
diff --git a/_pages/documentation/general_docs/building/index.md b/_pages/documentation/general_docs/building/index.md
index 672dceb..3482605 100644
--- a/_pages/documentation/general_docs/building/index.md
+++ b/_pages/documentation/general_docs/building/index.md
@@ -38,7 +38,7 @@
 ```
 sudo apt install build-essential git m4 scons zlib1g zlib1g-dev \
     libprotobuf-dev protobuf-compiler libprotoc-dev libgoogle-perftools-dev \
-    python-dev python libboost-all-dev
+    python-dev python-six python libboost-all-dev
 ```
 
 This will create an environment which uses Python 2.