[pygobject] docs: update homebrew install instructions

[Christoph Reiter <creiter src gnome org>]

commit 064a9baef0c9df7ae0cf3cace6d405bf78b01241
Author: Christoph Reiter <reiter christoph gmail com>
Date:   Fri Apr 13 21:08:08 2018 +0200

    docs: update homebrew install instructions
    
    Things default to Python3 now

 docs/getting_started.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/docs/getting_started.rst b/docs/getting_started.rst
index f3e746bd..4cee81af 100644
--- a/docs/getting_started.rst
+++ b/docs/getting_started.rst
@@ -102,7 +102,7 @@ and their dependencies. Follow the instructions for your platform below.
 
 1) Go to https://brew.sh/ and install homebrew
 2) Open a terminal
-3) Execute ``brew install pygobject3 --with-python3 gtk+3`` to install for both python2 and python3
+3) Execute ``brew install pygobject3 --with-python@2 gtk+3`` to install for both python2 and python3
 4) Change the directory to where your ``hello.py`` script can be found (e.g. ``cd Desktop``)
 5) Run ``python3 hello.py``