gn: Use the term 'out_dir' consistently when referring to the output directory.

tools/gn/command_ls.cc

 // Copyright 2014 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.
 
 #include <algorithm>
 #include <set>
 
 #include "base/command_line.h"
 #include "tools/gn/commands.h"
 #include "tools/gn/label_pattern.h"
 #include "tools/gn/setup.h"
 #include "tools/gn/standard_out.h"
 #include "tools/gn/target.h"
 
 namespace commands {
 
 const char kLs[] = "ls";
 const char kLs_HelpShort[] =
     "ls: List matching targets.";
 const char kLs_Help[] =
-    "gn ls <build dir> [<label_pattern>] [--out] [--all-toolchains]\n"
+    "gn ls <out_dir> [<label_pattern>] [--out] [--all-toolchains]\n"
     "\n"
     "  Lists all targets matching the given pattern for the given builn\n"
     "  directory. By default, only targets in the default toolchain will\n"
     "  be matched unless a toolchain is explicitly supplied.\n"
     "\n"
     "  If the label pattern is unspecified, list all targets. The label\n"
     "  pattern is not a general regular expression (see\n"
     "  \"gn help label_pattern\"). If you need more complex expressions,\n"
     "  pipe the result through grep.\n"
     "\n"
@@ skipping 78 matching lines @@
       OutputString(match.GetUserVisibleName(
           match.GetToolchainLabel() != default_tc_label));
       OutputString("\n");
     }
   }
 
   return 0;
 }
 
 }  // namespace commands