Fix async prefix in examples

Summary: related to T15996

Reviewers: iljazovic

Reviewed By: iljazovic

Subscribers: miljen

Differential Revision: https://repo.mireo.local/D33773

example/receiver.cpp

@@ -29,7 +29,7 @@
 struct config {
     std::string brokers = "broker.hivemq.com";
     uint16_t port = 1883;
-    std::string client_id = "async_mqtt5_tester";
+    std::string client_id = "boost_mqtt5_tester";
 };
 
 // Modified completion token that will prevent co_await from throwing exceptions.