docs(rds): Fix storage allocation units (#20692)

The upstream documentation
<https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-rds-database-instance.html#cfn-rds-dbinstance-allocatedstorage>
uses "gigabytes" in one place and "GiB" everywhere else, including the
dashboard. Presumably "gibibytes" was meant, so the CDK documentation
should reflect that.

Closes #20676.

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packages/@aws-cdk/aws-rds/lib/instance.ts

@@ -813,7 +813,7 @@ export interface DatabaseInstanceSourceProps extends DatabaseInstanceNewProps {
   readonly timezone?: string;
 
   /**
-   * The allocated storage size, specified in gigabytes (GB).
+   * The allocated storage size, specified in gibibytes (GiB).
    *
    * @default 100
    */