[SyncFS] Fix schema comment

The line was not consistent with the actual string defined in drive_backend_constants.cc.

BUG=None
TEST=None
NOTRY=true

Review URL: https://codereview.chromium.org/386343002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@282905 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/sync_file_system/drive_backend/metadata_database_index_on_disk.cc

@@ -46,7 +46,7 @@
 //   value: <int64 'app_root_tracker_id'>
 //
 //   # Index from file ID to the active tracker ID
-//   key: "ACTIVE_BY_FILE: " + <string 'file_id'>
+//   key: "ACTIVE_FILE: " + <string 'file_id'>
 //   value: <int64 'active_tracker_id'>
 //
 //   # Index from file ID to a tracker ID