swr: fix copy & pasted comment to match the code.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libswresample/swresample_internal.h

@@ -62,7 +62,7 @@ struct SwrContext {
     AudioData preout;                               ///< pre-output audio data: used for rematrix/resample
     AudioData out;                                  ///< converted output audio data
     AudioData in_buffer;                            ///< cached audio data (convert and resample purpose)
-    AudioData dither;                               ///< cached audio data (convert and resample purpose)
+    AudioData dither;                               ///< noise used for dithering
     int in_buffer_index;                            ///< cached buffer position
     int in_buffer_count;                            ///< cached buffer length
     int resample_in_constraint;                     ///< 1 if the input end was reach before the output end, 0 otherwise