Doxygen: fix documentation of rate_ctr_for_each_counter()

Doxygen was confused by duplicated documentation for both
definition and declaration of rate_ctr_for_each_counter().
Moreover, both variants contained some mistakes.

Let's avoid this duplication and keep the only (corrected) one.

Change-Id: Icca2d4a95bd5f96ae85a86909ec90fb8677cacf3
diff --git a/src/rate_ctr.c b/src/rate_ctr.c
index c9319a6..82fddc6 100644
--- a/src/rate_ctr.c
+++ b/src/rate_ctr.c
@@ -385,7 +385,7 @@
 }
 
 /*! Iterate over each counter in group and call function
- *  \param[in] counter group over whose counter to iterate
+ *  \param[in] ctrg counter group over which to iterate
  *  \param[in] handle_counter function pointer
  *  \param[in] data Data to hand transparently to \ref handle_counter
  *  \returns 0 on success; negative otherwise