avfilter/af_compand: cosmetics and doxy comment from libavfilter/af_compand_fork.c

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

libavfilter/af_compand.c

@@ -23,6 +23,11 @@
  *
  */
 
+/**
+ * @file
+ * audio compand filter
+ */
+
 #include "libavutil/avassert.h"
 #include "libavutil/avstring.h"
 #include "libavutil/opt.h"