Clarify the directory path in Build a Case Converter lesson 15

Lesson 15 of Build a Case Converter currently says:

> Within the `case_converter/` directory, create the entry point file `index.js`.

This instruction should include the parent directory, `build-a-case-converter/`, so the expected location is clear at this step.

In [this forum report](https://forum.freecodecamp.org/t/build-a-case-converter-build-a-case-converter-step-15/799587), the screenshot shows `case_converter/index.js` directly under the workspace root. The test correctly expects `build-a-case-converter/case_converter/index.js`. I reproduced the failure with the screenshot's layout and confirmed that an empty file at the expected path passes.

Update the description to say:

> Within the `build-a-case-converter/case_converter/` directory, create the entry point file `index.js`.

The file is [curriculum/locales/english/build-a-case-converter.md](https://github.com/freeCodeCamp/back-end-development-and-apis/blob/main/curriculum/locales/english/build-a-case-converter.md), under `## 15`. The test already checks the correct path.

Open curriculum/locales/english/build-a-case-converter.md and locate section ## 15. Update the directory path in the instruction to include build-a-case-converter/case_converter/, then run the existing lesson test to confirm it still checks the expected path.

Written by the indexing model from the issue text.