Revert some silly renamings that leaked in from a pull.

doc/build_system.txt

@@ -9,7 +9,7 @@ V
 
 DESTDIR
     Destination directory for the install targets, useful to prepare packages
-    or install Libav in cross-environments.
+    or install FFmpeg in cross-environments.
 
 Makefile targets:
 

libavcodec/arm/ac3dsp_init_arm.c

 /*
  * Copyright (c) 2011 Mans Rullgard <mans@mansr.com>
  *
- * This file is part of Libav.
+ * This file is part of FFmpeg.
  *
- * Libav is free software; you can redistribute it and/or
+ * FFmpeg is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
  * License as published by the Free Software Foundation; either
  * version 2.1 of the License, or (at your option) any later version.
  *
- * Libav is distributed in the hope that it will be useful,
+ * FFmpeg is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  * Lesser General Public License for more details.
  *
  * You should have received a copy of the GNU Lesser General Public
- * License along with Libav; if not, write to the Free Software
+ * License along with FFmpeg; if not, write to the Free Software
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */