Fix some wording in the IO documentation (#7111)

Author: cebernard

lib/elixir/lib/io.ex

@@ -27,9 +27,9 @@ defmodule IO do
     * `:stderr` - a shortcut for the named process `:standard_error`
       provided in Erlang
 
-  IO devices maintain their position, that means subsequent calls to any
-  reading or writing functions will start from the place when the device
-  was last accessed. Position of files can be changed using the
+  IO devices maintain their position, which means subsequent calls to any
+  reading or writing functions will start from the place where the device
+  was last accessed. The position of files can be changed using the
   `:file.position/2` function.
 
   """