Fix early return in NativeKafka#start (#437)

I think this was a typo, and could potentially lead to creating multiple polling threads.

Author: ferrous26

lib/rdkafka/native_kafka.rb

@@ -37,7 +37,7 @@ def initialize(inner, run_polling_thread:, opaque:, auto_start: true)
 
     def start
       synchronize do
-        @return if @started
+        return if @started
 
         @started = true