Update README.md

Author: josemoracard

exercises/04-Call-a-function/README.md

@@ -38,7 +38,7 @@ area = calculate_area(3,6)
 square_area1 = calculate_area(4,4)
 ```
 
-![squares](http://i.imgur.com/VyoJRAL.png)
+![Squares](http://i.imgur.com/VyoJRAL.png)
 
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