Fix findIndex documentation.

src/lib/es2015.core.d.ts

@@ -13,7 +13,7 @@ interface Array<T> {
     find(predicate: (value: T, index: number, obj: Array<T>) => boolean, thisArg?: any): T | undefined;
 
     /**
-      * Returns the index of the first element in the array where predicate is true, and undefined
+      * Returns the index of the first element in the array where predicate is true, and -1
       * otherwise.
       * @param predicate find calls predicate once for each element of the array, in ascending
       * order, until it finds one where predicate returns true. If such an element is found,