chore(docs): fix invalid anchor links in DESIGN_GUIDELINES (#31997)

### Issue # (if applicable)

n/A

### Description of changes

Fix invalid anchor links in [DESIGN_GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md).


### 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)

----

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

Author: ren-yamanashi

docs/DESIGN_GUIDELINES.md

@@ -9,7 +9,7 @@ the AWS Construct Library in order to ensure a consistent and integrated
 experience across the entire AWS surface area.
 
 * [Preface](#preface)
-* [What's Included](#what-s-included)
+* [What's Included](#whats-included)
 * [API Design](#api-design)
   * [Modules](#modules)
   * [Construct Class](#construct-class)
@@ -66,7 +66,7 @@ experience across the entire AWS surface area.
   * [Unit tests](#unit-tests)
   * [Integration tests](#integration-tests)
   * [Versioning](#versioning)
-* [Naming & Style](#naming---style)
+* [Naming & Style](#naming--style)
   * [Naming Conventions](#naming-conventions)
   * [Coding Style](#coding-style)