licensing: fix argparse usage info

We want the module docstrings to be the parser description, not the
usage.  The usage contains info about the valid options and argparse
generates it automatically for us.  It also matches what we do by
default everywhere else.

BUG=None
TEST=ran tools with --help and with bad usage

Change-Id: I18701e88de8be22ab382c33dcffdf0f7ce31f0ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2309719
Auto-Submit: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Alex Klein <saklein@chromium.org>
Commit-Queue: Alex Klein <saklein@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/diff_license_html.py b/scripts/diff_license_html.py
index 3d017b1..d0d31f4 100644
--- a/scripts/diff_license_html.py
+++ b/scripts/diff_license_html.py
@@ -114,7 +114,7 @@
 
 
 def main(args):
-  parser = commandline.ArgumentParser(usage=__doc__)
+  parser = commandline.ArgumentParser(description=__doc__)
   parser.add_argument('html1', metavar='license1.html', type='path',
                       help='old html file')
   parser.add_argument('html2', metavar='license2.html', type='path',