Fix a typo in GN documentation

tools/gn/substitution_writer.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 "tools/gn/substitution_writer.h"
 
 #include "tools/gn/build_settings.h"
 #include "tools/gn/escape.h"
 #include "tools/gn/filesystem_utils.h"
 #include "tools/gn/output_file.h"
@@ skipping 80 matching lines @@
       source file is in a different directory than the BUILD.gn file.
         "//foo/bar/baz.txt" => "gen/foo/bar"
 
   {{source_out_dir}}
       The object file directory (*) corresponding to the source file's path,
       relative to the build directory. this us be different than the target's
       out directory if the source file is in a different directory than the
       build.gn file.
         "//foo/bar/baz.txt" => "obj/foo/bar"
 
-  {{source_target_relative}}\n"
+  {{source_target_relative}}
       The path to the source file relative to the target's directory. This will
       generally be used for replicating the source directory layout in the
       output directory. This can only be used in actions and it is an error to
       use in process_file_template where there is no "target".
         "//foo/bar/baz.txt" => "baz.txt"
 
 (*) Note on directories
 
   Paths containing directories (except the source_root_relative_dir) will be
   different depending on what context the expansion is evaluated in. Generally
@@ skipping 489 matching lines @@
         return tool->default_output_extension();
       if (target->output_extension().empty())
         return std::string();  // Explicitly set to no extension.
       return std::string(".") + target->output_extension();
 
     default:
       NOTREACHED();
       return std::string();
   }
 }