Update article.md

Gorgan Razvan Florin · web-flow · commit c0c203e4cbfa · 2017-04-20T20:57:25.000+03:00
diff --git a/1-js/05-data-types/07-map-set-weakmap-weakset/article.md b/1-js/05-data-types/07-map-set-weakmap-weakset/article.md
@@ -167,7 +167,7 @@ recipeMap.forEach( (value, key, map) => {
 
 ## Set
 
-`Set` -- is a collection of values, where each value may occur.
+`Set` -- is a collection of values, where each value may occur only once.
 
 The main methods are:
 
