docs(ecr): capitalize Import in existing repository header (#35787)

### Reason for this change



Headers should follow proper title case formatting

### Description of changes



Capitalized "Import" in the "Import existing repository" header in ECR README



### 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)

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

Author: ozelalisen

packages/aws-cdk-lib/aws-ecr/README.md

@@ -248,7 +248,7 @@ repository.addToResourcePolicy(new iam.PolicyStatement({
 }));
 ```
 
-## import existing repository
+## Import existing repository
 
 You can import an existing repository into your CDK app using the `Repository.fromRepositoryArn`, `Repository.fromRepositoryName` or `Repository.fromLookup` method.
 These methods take the ARN or the name of the repository and returns an `IRepository` object.