fix(types): Make element.then()'s signature more broad. (#3719)

Author: heathkit

lib/element.ts

@@ -479,7 +479,7 @@ export class ElementArrayFinder extends WebdriverWebElement {
    * @returns {!webdriver.promise.Promise} A promise which will resolve to
    *     an array of ElementFinders represented by the ElementArrayFinder.
    */
-  then(fn?: (value: any) => {} | wdpromise.IThenable<{}>, errorFn?: (error: any) => any):
+  then(fn?: (value: any) => any | wdpromise.IThenable<any>, errorFn?: (error: any) => any):
       wdpromise.Promise<any[]> {
     if (this.actionResults_) {
       return this.actionResults_.then(fn, errorFn);