Use - instead of _ for AMR options in help output.

patch by Benoit Fouet, benoit.fouet purplelabs com

Originally committed as revision 8313 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -99,10 +99,10 @@ show_help(){
   echo "  --enable-x264            enable H.264 encoding via x264 [default=no]"
   echo "  --enable-xvid            enable Xvid encoding via xvidcore,"
   echo "                           native MPEG-4/Xvid encoder exists [default=no]"
-  echo "  --enable-amr_nb          enable amr_nb float audio codec"
-  echo "  --enable-amr_nb-fixed    use fixed point for amr-nb codec"
-  echo "  --enable-amr_wb          enable amr_wb float audio codec"
-  echo "  --enable-amr_if2         enable amr_wb IF2 audio codec"
+  echo "  --enable-amr-nb          enable amr-nb float audio codec"
+  echo "  --enable-amr-nb-fixed    use fixed point for amr-nb codec"
+  echo "  --enable-amr-wb          enable amr-wb float audio codec"
+  echo "  --enable-amr-if2         enable amr-wb IF2 audio codec"
   echo ""
   echo "Advanced options (experts only):"
   echo "  --source-path=PATH       path to source code [$source_path]"