fix bad python-gitlab instructions

For installing prerequisites the python-gitlab doc was missing
the install command.

docs/prerequisites.md

@@ -18,7 +18,7 @@ python-gitlab
     git clone https://github.com/Itxaka/python-gitlab.git
     cd python-gitlab
     git checkout 9c5e375599a6d89ab1f4520224f47b43b40bcf9b
-    python setup.py
+    python setup.py install
 
 You can find the source to git at the [git-core project][5].  For instructions on other platforms see the [Getting Started - Installing Git section of the git book][6].