fix Iterator.concat signature in docs

Author: zloirock

README.md

@@ -2691,7 +2691,7 @@ core-js(-pure)/stage/3
 Module [`esnext.iterator.concat`](https://github.com/zloirock/core-js/blob/master/packages/core-js/modules/esnext.iterator.concat.js)
 ```ts
 class Iterator {
-  concat(...items: Array<IterableObject>): Iterator<any>;
+  static concat(...items: Array<IterableObject>): Iterator<any>;
 }
 ```
 [*CommonJS entry points:*](#commonjs-api)

docs/web/docs/features/proposals/iterator-sequencing.md

@@ -8,7 +8,7 @@
 ## Built-ins signatures
 ```ts
 class Iterator {
-  concat(...items: Array<IterableObject>): Iterator<any>;
+  static concat(...items: Array<IterableObject>): Iterator<any>;
 }
 ```