avutil/channel_layout: AV_CH_LAYOUT_6POINT1_BACK not reachable in parsing

Trying to make heads and tails out of DTS 6.1 I can across this typo.

I also noticed that this wiki page is incorrect or misleading, the
channel order for 6.1 given does not match the source code.  At the
least it should be clarified that the layout given does not apply to
DTS.  https://trac.ffmpeg.org/wiki/AudioChannelManipulationSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavutil/channel_layout.c

@@ -94,7 +94,7 @@ static const struct {
     { "6.0(front)",  6,  AV_CH_LAYOUT_6POINT0_FRONT },
     { "hexagonal",   6,  AV_CH_LAYOUT_HEXAGONAL },
     { "6.1",         7,  AV_CH_LAYOUT_6POINT1 },
-    { "6.1",         7,  AV_CH_LAYOUT_6POINT1_BACK },
+    { "6.1(back)",   7,  AV_CH_LAYOUT_6POINT1_BACK },
     { "6.1(front)",  7,  AV_CH_LAYOUT_6POINT1_FRONT },
     { "7.0",         7,  AV_CH_LAYOUT_7POINT0 },
     { "7.0(front)",  7,  AV_CH_LAYOUT_7POINT0_FRONT },