Update service-level-indicators/spark-retrieval-success-rate.md to describe how Spark v1.5 ingests DDO deals

[bajtos]

Update https://github.com/filecoin-project/service-classes/blob/main/service-level-indicators/spark-retrieval-success-rate.md#deal-ingestion to describe the new spark-deal-observer ingestion pipeline.

[bajtos]

It seems that somebody contributed the docs:

• Update service-level-indicators/spark-retrieval-success-rate.md to describe how Spark v1.5 ingests DDO deals #238 filecoin-project/service-classes#13