lavu/adler32: Fix doxy group definition, take two

The description if for the function, not the group.
Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/adler32.h

@@ -28,6 +28,9 @@
  * @defgroup lavu_adler32 Adler32
  * @ingroup lavu_crypto
  * @{
+ */
+
+/**
  * Calculate the Adler32 checksum of a buffer.
  *
  * Passing the return value to a subsequent av_adler32_update() call