Fix typo

Author: knownasilya

README.md

@@ -7,7 +7,7 @@
 This adds more guard rails for Ember. Just because something "works" does not mean it is correct or in line with the framework's programming model. These footguns include:
 
 - Not adhereing to Data Down Actions Up
-- Side-efffect programming
+- Side-effect programming
 - Usage of observers
 - Usage of attrs.*
 - Computed properties without dependent keys