coap_dtls.h: Fix doxygen warning

This change removes a doxygen comment starter because doxygen is
getting confused otherwise because of the immediately preceding
documentation group start.

include/coap2/coap_dtls.h

@@ -465,7 +465,7 @@ typedef struct coap_dtls_spsk_t {
  * @{
  */
 
-/**
+/*
  * Creates a new DTLS context for the given @p coap_context. This function
  * returns a pointer to a new DTLS context object or @c NULL on error.
  *