ac3enc: fix AC3 downmix metadata issue

Due to a typo, it was impossible to write 0.595 / -4.5 dB
of ltrt_cmixlev, ltrt_surmixlev, loro_cmixlev, loro_surmixlev.
Without any error 0.841 / -1.5 dB was written to file.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/ac3enc.c

@@ -63,7 +63,7 @@ static const float surmixlev_options[SURMIXLEV_NUM_OPTIONS] = {
 
 #define EXTMIXLEV_NUM_OPTIONS 8
 static const float extmixlev_options[EXTMIXLEV_NUM_OPTIONS] = {
-    LEVEL_PLUS_3DB,  LEVEL_PLUS_1POINT5DB,  LEVEL_ONE,       LEVEL_MINUS_4POINT5DB,
+    LEVEL_PLUS_3DB,  LEVEL_PLUS_1POINT5DB,  LEVEL_ONE,       LEVEL_MINUS_1POINT5DB,
     LEVEL_MINUS_3DB, LEVEL_MINUS_4POINT5DB, LEVEL_MINUS_6DB, LEVEL_ZERO
 };