fix: typo (#4791)

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server/README-MQTT.md

@@ -142,7 +142,7 @@ Here is the overview of how we set up and use JetStream **streams**,
 ## JetStream API
 
 All interactions with JetStream are performed via `mqttJSA` that sends NATS
-requests to JetStream. Most are processed syncronously and await a response,
+requests to JetStream. Most are processed synchronously and await a response,
 some (e.g. `jsa.sendAck()`) are sent asynchronously. JetStream API is usually
 referred to as `jsa` in the code. No special locking is required to use `jsa`,
 however the asynchronous use of JetStream may create race conditions with