docs/MQTT: replace confusing 80 by 75

I was a bit surprised by the `80`: first thought: what's HTTP doing
here? ;)

Closes #5236
diff --git a/docs/MQTT.md b/docs/MQTT.md
index 1c7b678..a409714 100644
--- a/docs/MQTT.md
+++ b/docs/MQTT.md
@@ -31,7 +31,7 @@
 
 Example publish:
 
-    curl -d 80 mqtt://host/home/bedroom/dimmer
+    curl -d 75 mqtt://host/home/bedroom/dimmer
 
 ## What does curl deliver as a response to a subscribe