Fix small typo in common.py

This causes a crash if running verbose mode

Change-Id: Ib5fe3dada8c81874aae110e21252edad05668a36
Reviewed-on: https://pdfium-review.googlesource.com/32737
Reviewed-by: Henrique Nakashima <hnakashima@chromium.org>
Commit-Queue: Henrique Nakashima <hnakashima@chromium.org>
Commit-Queue: Ryan Harrison <rharrison@chromium.org>

testing/tools/common.py

diff --git a/testing/tools/common.py b/testing/tools/common.py
index fc16004..da3cac8 100755
--- a/testing/tools/common.py
+++ b/testing/tools/common.py
@@ -141,7 +141,7 @@
   os.chdir(cwd)
   arg_match_output = re.search('%s = (.*)' % arg_name, gn_args_output).group(1)
   if verbose:
-    print >> sys.stderr, "Found '%s' for value of %s" % (arg_match_output, arg)
+    print >> sys.stderr, "Found '%s' for value of %s" % (arg_match_output, arg_name)
   return arg_match_output == 'true'