[SPARK-52947][SDP] Fix image path in declarative pipelines programming guide

### What changes were proposed in this pull request?

Fixes the path of the diagram in the declarative pipelines programming guide to point to the right image file.

### Why are the changes needed?

The image is not rendering correctly.

### Does this PR introduce _any_ user-facing change?

No

### How was this patch tested?

Manual inspection: https://github.com/sryza/spark/blob/declarative-pipelines-img/docs/declarative-pipelines-programming-guide.md

### Was this patch authored or co-authored using generative AI tooling?

Closes #51657 from sryza/declarative-pipelines-img.

Authored-by: Sandy Ryza <[email protected]>
Signed-off-by: Hyukjin Kwon <[email protected]>

Author: sryza

docs/declarative-pipelines-programming-guide.md

@@ -33,7 +33,7 @@ SDP is designed for both batch and streaming data processing, supporting common
 
 The key advantage of SDP is its declarative approach - you define what tables should exist and what their contents should be, and SDP handles the orchestration, compute management, and error handling automatically.
 
-![Dataflow Graph](../img/declarative-pipelines-dataflow-graph.png)
+![Dataflow Graph](img/declarative-pipelines-dataflow-graph.png)
 
 ## Key Concepts