Update activity.md

Author: amelinagzz

Lessons/09-Functional-ProgrammingPt.1/assignments/activity.md

@@ -68,3 +68,6 @@ let cities = [paris, madrid, amsterdam, berlin]
 ```
 
 Print a list of cities with at least one million inhabitants, together with their total populations.
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+*From: Chris Eidhof. “Functional Swift.” Apple Books*