chore: fix missing substitution for custom_prompt_lambda_arn

Author: Daniel Lorch

patterns/pattern-2/template.yaml

@@ -1026,7 +1026,7 @@ Resources:
                 order: 7
               custom_prompt_lambda_arn:
                 type: string
-                description: "(Optional) ARN of a Lambda function to generate custom extraction prompts. Function name must start with 'GENAIIDP-'. If not provided, default prompts will be used. The Lambda function receives the complete config, prompt placeholders, default task prompt content, and serialized document, and returns custom system_prompt and task_prompt_content. Example: arn:${AWS::Partition}:lambda:us-east-1:123456789012:function:GENAIIDP-my-extractor"
+                description: !Sub "(Optional) ARN of a Lambda function to generate custom extraction prompts. Function name must start with 'GENAIIDP-'. If not provided, default prompts will be used. The Lambda function receives the complete config, prompt placeholders, default task prompt content, and serialized document, and returns custom system_prompt and task_prompt_content. Example: arn:${AWS::Partition}:lambda:us-east-1:123456789012:function:GENAIIDP-my-extractor"
                 order: 8
           assessment:
             order: 5