docs(opensearchservice): fix broken Markdown due to an invisible char (#31007)

### Issue # (if applicable)

Closes #<issue number here>.

### Reason for this change

For some invisible char in the Markdown, it wasn't rendered correctly.

### Description of changes

I am unsure what was there, but replacing the "space" between `##` and `Enable` with a regular space (i.e., ' ') fixed it.

### Description of how you validated changes

After this change, using the Markdown preview feature in my editor, as well as regular Markdown syntax highlighting, worked as expected.

### Checklist
- [X] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: exalted

packages/aws-cdk-lib/aws-opensearchservice/README.md

@@ -464,7 +464,7 @@ const domain = new Domain(this, 'Domain', {
 });
 ```
 
-## Enable support for Multi-AZ with Standby deployment
+## Enable support for Multi-AZ with Standby deployment
 
 The domain can be configured to use [multi-AZ with standby](https://docs.aws.amazon.com/opensearch-service/latest/developerguide/managedomains-multiaz.html#managedomains-za-standby).