Updates pre_course_MAC.md

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Author: Tania Allard

pre_course_MAC.md

@@ -6,7 +6,7 @@ There are a few things you'll need to get set up before our first class, so that
 
 Things we need to do (we'll explain each of them in these notes):
 
-1. Check Python's installed
+1. Check Python is installed
 2. Install pip
 3. Install a text editor
 4. Make sure you've got a GitHub account