Cosmetic: fix arg desc of db_subscr_update_msisdn_by_imsi()

Properly note that NULL can only be passed as msisdn, not imsi in that
function.

Change-Id: I19b6ad0cf6e9a4bfa9bffa447ebfc7bd1bcac361
diff --git a/src/db_hlr.c b/src/db_hlr.c
index c97cd82..2569477 100644
--- a/src/db_hlr.c
+++ b/src/db_hlr.c
@@ -141,8 +141,8 @@
 
 /*! Set a subscriber's MSISDN in the HLR database.
  * \param[in,out] dbc  database context.
- * \param[in] imsi  ASCII string of IMSI digits, or NULL to remove the MSISDN.
- * \param[in] msisdn  ASCII string of MSISDN digits.
+ * \param[in] imsi  ASCII string of IMSI digits
+ * \param[in] msisdn  ASCII string of MSISDN digits, or NULL to remove the MSISDN.
  * \returns 0 on success, -EINVAL in case of invalid MSISDN string, -EIO on
  *          database failure, -ENOENT if no such subscriber exists.
  */