Fix misspelling in txt_to_polymer_grdp.py.

tools/polymer/txt_to_polymer_grdp.py

 #!/usr/bin/env python
 # Copyright 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 from __future__ import with_statement
 import os
 import string
 import sys
 
 
 FILE_TEMPLATE = \
 """<?xml version="1.0" encoding="utf-8"?>
 <!--
   This file is generated.
   Please use 'src/tools/polymer/polymer_grdp_to_txt.py' and
   'src/tools/polymer/txt_to_polymer_grdp.py' to modify it, if possible.
 
-  'polymer_grdp_to_txt.py' converts 'polymer_resources.grdp' to a plane list of
+  'polymer_grdp_to_txt.py' converts 'polymer_resources.grdp' to a plain list of
   used Polymer components:
     ...
     iron-iron-iconset/iron-iconset-extracted.js
     iron-iron-iconset/iron-iconset.html
     ...
 
   'txt_to_polymer_grdp.py' converts list back to GRDP file.
 
   Usage:
     $ polymer_grdp_to_txt.py polymer_resources.grdp > /tmp/list.txt
@@ skipping 47 matching lines @@
     if not os.path.exists(full_path):
       raise FileNotFoundException('%s not found' % full_path)
 
     template = DEFINITION_TEMPLATE_1_0
     lines['v_1_0'].append(
         template % (PathToGritId(path), path))
   print FILE_TEMPLATE % { 'v_1_0': '\n'.join(lines['v_1_0']) }
 
 if __name__ == '__main__':
   sys.exit(main(sys.argv))