lavu/adler32: Fix doxy group definition

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/adler32.h

@@ -25,7 +25,9 @@
 #include "attributes.h"
 
 /**
+ * @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
@@ -40,4 +42,8 @@
 unsigned long av_adler32_update(unsigned long adler, const uint8_t *buf,
                                 unsigned int len) av_pure;
 
+/**
+ * @}
+ */
+
 #endif /* AVUTIL_ADLER32_H */