py: fix typo in webdriver.py. fix #850

Author: ddavison

py/selenium/webdriver/remote/webdriver.py

@@ -236,7 +236,7 @@ def find_elements_by_id(self, id_):
          - id\_ - The id of the elements to be found.
 
         :Usage:
-            driver.find_element_by_id('foo')
+            driver.find_elements_by_id('foo')
         """
         return self.find_elements(by=By.ID, value=id_)