Entrypoint trailling slash is stripped from original entrypoint fetch.

**API doc parser version(s) affected**: 0.15.1

**Description**

Since the entrypoint prefix is stripped [here](https://github.com/api-platform/api-doc-parser/blob/de28a3c6bb9644ea2c6549513bdf7c411317664d/src/hydra/parseHydraDocumentation.ts#L202) it means that the request to fetch the API doc is made to the URL without this slash.

It means that in the case were the API only replies to the URL with the slash the request will fails and the admin won't load.

**How to reproduce**

Create an Admin with an entrypoint that ends with a slash and an API that returns a error when the request is done without this slash.

**Possible Solution**

Since the slash is removed intentionally we need to keep it, but maybe we could keep the original supplied URL for the original fetch of the entry point.