[hotfix] Improve FlinkDeployment spec overview doc (#788)

Author: caicancai

Diff for: docs/content/docs/custom-resource/overview.md

@@ -85,7 +85,7 @@ The spec contains all the information the operator need to deploy and manage you
 
 Most deployments will define at least the following fields:
  - `image` : Docker used to run Flink job and task manager processes
- - `flinkVersion` : Flink version used in the image (`v1_15`, `v1_16` ...)
+ - `flinkVersion` : Flink version used in the image (`v1_15`, `v1_16`, `v1_17`, `v1_18`, ...)
  - `serviceAccount` : Kubernetes service account used by the Flink pods
  - `taskManager, jobManager` : Job and Task manager pod resource specs (cpu, memory, ephemeralStorage)
  - `flinkConfiguration` : Map of Flink configuration overrides such as HA and checkpointing configs