docs: fix wrong annotation used in example (#20964)

Used a misleading `@mutating` annotation instead of a `@config` annotation.


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docs/DESIGN_GUIDELINES.md

@@ -754,10 +754,10 @@ interface IFoo extends IConstruct {
 class Foo extends Construct implements IFoo {
   public bar() { }
 
-  /** @mutating */
+  @config
   public goo() { }
 
-  public mutateMe() { } // ERROR! missing "@mutating" or missing on IFoo
+  public mutateMe() { } // ERROR! missing "@config" or missing on IFoo
 }
 ```