Fix format of spark architecture page

Author: hbxie

docs/backend/spark-architecture.md

@@ -15,6 +15,7 @@ The red lines are the path for the queries that come in through the PubSub, the
 ### Data processing
 
 Bullet can accept arbitrary sources of data as long as they can be ingested by Spark. They can be Kafka, Flume, Kinesis, and TCP sockets etc. In order to hook up your data to Bullet Spark, you just need to implement the [Data Producer Trait](https://github.com/bullet-db/bullet-spark/blob/master/src/main/scala/com/yahoo/bullet/spark/DataProducer.scala). In your implementation, you can either:
+
 * Use [Spark Streaming built-in sources](https://spark.apache.org/docs/latest/streaming-programming-guide.html#input-dstreams-and-receivers) to receive data. Below is a quick example for a direct Kafka source in Scala. You can also write it in Java:
 
 ```scala