Fix mistake

compare.md

@@ -35,7 +35,7 @@ interface Comparator<T> {
 ```java
 class SortByHouseNumber implements Comparator<Adress> {
 	public int compare(Adress a1, Adress a2){
-		return a.getHouseNumber - b.getHouseNumber;
+		return a1.getHouseNumber - a2.getHouseNumber;
 	}
 }
 ```