mdns: Fix assertion message in mdns_resp_add_service_txtitem()

So we know which function emits the assertion.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
diff --git a/src/apps/mdns/mdns.c b/src/apps/mdns/mdns.c
index 92f7a2c..c140185 100644
--- a/src/apps/mdns/mdns.c
+++ b/src/apps/mdns/mdns.c
@@ -2019,7 +2019,7 @@
 err_t
 mdns_resp_add_service_txtitem(struct mdns_service *service, const char *txt, u8_t txt_len)
 {
-  LWIP_ASSERT("mdns_resp_add_service: service != NULL", service);
+  LWIP_ASSERT("mdns_resp_add_service_txtitem: service != NULL", service);
 
   /* Use a mdns_domain struct to store txt chunks since it is the same encoding */
   return mdns_domain_add_label(&service->txtdata, txt, txt_len);