sys/hashes: fix doxygen group

- fix group membership
- improve description style

sys/hashes/doc.txt

@@ -10,7 +10,7 @@
  * @defgroup    sys_hashes Hashes
  * @ingroup     sys
  *
- * @brief       RIOT provides a collection of hash algorithms.
+ * @brief       A collection of hash algorithms.
  *
  * RIOT supports the following hash functions:
  *

sys/include/hashes.h

@@ -7,9 +7,7 @@
  */
 
 /**
- * @ingroup     sys_hashes Hashes
- * @ingroup     sys
- * @brief       Hash function library
+ * @ingroup     sys_hashes
  * @{
  *
  * @file