fix(test-app): fix spelling error (#34762)

### Issue # (if applicable)

N/A

### Reason for this change

Typographical error in the error message string — "lenght" was misspelled and needed correction.

### Description of changes

- Fixed a typo in the `MultipleFileAssetsApp` class.
- Updated the error message to correct the word "lenght" to "length".
- No logic or functional changes were made.
- Alternatives such as rephrasing the message were considered unnecessary.

### Describe any new or updated permissions being added

N/A

### Description of how you validated changes

- Visually verified the corrected string in the source code.
- Confirmed that no logic or behavior was affected.
- No tests were necessary since this was a non-functional change.

### 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: Einherjar1632

packages/aws-cdk-lib/pipelines/test/testhelpers/test-app.ts

@@ -204,7 +204,7 @@ export class MultipleFileAssetsApp extends Stage {
 
     const fileNames = ['test-file-asset.txt', 'test-file-asset-two.txt', 'test-file-asset-three.txt'];
     if (props.displayNames && props.displayNames.length !== props.n) {
-      throw new Error('Incorrect displayNames lenght');
+      throw new Error('Incorrect displayNames length');
     }
 
     for (let i = 0; i < props.n; i++) {