tweak branch example

Author: alimanfoo

docs/contributing.rst

@@ -105,7 +105,8 @@ Creating a branch
 
 It's best to create a new, separate branch for each piece of work you want to do. E.g.::
 
-    git checkout -b shiny-new-feature
+    git fetch upstream
+    git checkout -b shiny-new-feature upsteam/master
 
 This changes your working directory to the shiny-new-feature branch. Keep any changes in
 this branch specific to one bug or feature so it is clear what the branch brings to