chore(s3): update README to mention support for uppercase letters in legacy bucket names (#31852)

### Issue # (if applicable)

Closes #<issue number here>.

### Reason for this change


PR #31813

### Description of changes




- update s3 README to mention support for uppercase letters in legacy bucket names

### Description of how you validated changes



preview of the README file

### 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: 5d

Diff for: packages/aws-cdk-lib/aws-s3/README.md

@@ -225,8 +225,8 @@ To import an existing bucket into your CDK application, use the `Bucket.fromBuck
 factory method. This method accepts `BucketAttributes` which describes the properties of an already
 existing bucket:
 
-Note that this method allows importing buckets with legacy names containing underscores (`_`), which was
-permitted for buckets created before March 1, 2018. For buckets created after this date, underscores
+Note that this method allows importing buckets with legacy names containing uppercase letters (`A-Z`) or underscores (`_`), which were
+permitted for buckets created before March 1, 2018. For buckets created after this date, uppercase letters and underscores
 are not allowed in the bucket name.
 
 ```ts