message: Update changelog with PR link for batch-produce
diff: ## Unreleased
  ## Platform Version 0.7.3 - 2020-04-02
  -* Added batching for producing records with `send_all` API.
  +* Added batching for producing records with `send_all` API ([#896](https://github.com/infinyon/fluvio/pull/896)).
  * WASM based Smart Stream Filter MVP ([#901](https://github.com/infinyon/fluvio/pull/901)).
  * Fix topic not being deleted when SPU goes offline ([#914](https://github.com/infinyon/fluvio/pull/914))
  