Update by.js: Add forgotten RelativeBy in check options (#12289)

Author: AdamPDotty

javascript/node/selenium-webdriver/lib/by.js

@@ -396,7 +396,7 @@ class RelativeBy {
  *     strategy.
  */
 function check(locator) {
-  if (locator instanceof By || typeof locator === 'function') {
+  if (locator instanceof By || locator instanceof RelativeBy || typeof locator === 'function') {
     return locator
   }