Fix OPT syntax error for Python3 usage

Author: bnmnetp

runestone/common/project_template/_sources/overview.rst

@@ -270,10 +270,10 @@ next line executed.
     x = 2
     y = 0
     if x % 2 == 1:
-        print 'x is odd'
+        print('x is odd')
         y = y + x
     else:
-        print 'x is even'
+        print('x is even')
         y = y - x
 
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