docs(iam): hints at accountId as string type (aws#22149)

----

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---

With the merge of [aws#20292](aws#20292) the accountID type was essentially required to be a string with the exception of an unencoded token. So, while the type of accountId cannot be set to string and must remain any to preserve compatibility, the hint for accountId should at least suggest that a string should be used rather than an int.

packages/@aws-cdk/aws-iam/lib/principals.ts

@@ -459,7 +459,7 @@ export class AccountPrincipal extends ArnPrincipal {
 
   /**
    *
-   * @param accountId AWS account ID (i.e. 123456789012)
+   * @param accountId AWS account ID (i.e. '123456789012')
    */
   constructor(public readonly accountId: any) {
     super(new StackDependentToken(stack => `arn:${stack.partition}:iam::${accountId}:root`).toString());
@@ -936,4 +936,4 @@ export function validateConditionObject(x: unknown): asserts x is Record<string,
   if (!x || typeof x !== 'object' || Array.isArray(x)) {
     throw new Error('A Condition should be represented as a map of operator to value');
   }
-}
+}