fix(docdb): DB Instance ARN uses 'docdb' as the service component ins…

…tead of 'rds' (aws#19555)

Fixes aws#19554

----

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### Adding new Unconventional Dependencies:

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### New Features

* [ ] Have you added the new feature to an [integration test](https://github.com/aws/aws-cdk/blob/master/INTEGRATION_TESTS.md)?
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packages/@aws-cdk/aws-docdb/lib/instance.ts

@@ -101,7 +101,7 @@ abstract class DatabaseInstanceBase extends cdk.Resource implements IDatabaseIns
    */
   public get instanceArn(): string {
     return cdk.Stack.of(this).formatArn({
-      service: 'docdb',
+      service: 'rds',
       resource: 'db',
       arnFormat: ArnFormat.COLON_RESOURCE_NAME,
       resourceName: this.instanceIdentifier,

packages/@aws-cdk/aws-docdb/test/instance.test.ts

@@ -117,7 +117,7 @@ describe('DatabaseInstance', () => {
           [
             'arn:',
             { Ref: 'AWS::Partition' },
-            ':docdb:us-test-1:12345:db:',
+            ':rds:us-test-1:12345:db:',
             { Ref: 'InstanceC1063A87' },
           ],
         ],
@@ -160,7 +160,7 @@ describe('DatabaseInstance', () => {
           [
             'arn:',
             { Ref: 'AWS::Partition' },
-            `:docdb:us-test-1:12345:db:${instanceIdentifier}`,
+            `:rds:us-test-1:12345:db:${instanceIdentifier}`,
           ],
         ],
       },