Update README.md

README.md

@@ -72,7 +72,7 @@ cd paho.mqtt.cpp
 cmake -Bbuild -H.
 sudo cmake --build build/ --target install
 ```
-### Installing MQTT Publisher/Subscriber demo
+## Installing MQTT Publisher/Subscriber demo
 open terminal, go to **mqtt_demo** directroy inside the project and run
 `cmake .`