Update copy by generation docs

lib/gcloud/storage/file.rb

@@ -229,6 +229,9 @@ def download path, options = {}
       #   path to copy the file to in the given bucket. (+String+)
       # +options+::
       #   An optional Hash for controlling additional behavior. (+Hash+)
+      # <code>options[:generation]</code>::
+      #   Select a specific revision of the file to copy. The default is the
+      #   latest version. (+Integer+)
       # <code>options[:acl]</code>::
       #   A predefined set of access controls to apply to new file.
       #   (+String+)
@@ -282,6 +285,7 @@ def download path, options = {}
       #
       #   file.copy "copy/of/previous/generation/file.ext",
       #             generation: 123456
+      #
       def copy dest_bucket_or_path, dest_path = nil, options = {}
         ensure_connection!
         dest_bucket, dest_path, options = fix_copy_args dest_bucket_or_path,