docs: correct GITHUB_CUSTOM_API_URL format (super-linter#1991)

In the README.md file the example value of `GITHUB_CUSTOM_API_URL` for usage on GHES ends in a `api/v3/` this is incorrect, instead `api/v3` needs to be used. Also it states that the defautl value is `api.github.com`, the true default value is `https://api.github.com`, this again is confusing whether or not the `https` needs to be included or not (it does need to be included)

Co-authored-by: Lukas Gravley <[email protected]>

README.md

@@ -285,7 +285,7 @@ But if you wish to select or exclude specific linters, we give you full control
 | **FILTER_REGEX_INCLUDE**           | `all`                           | Regular expression defining which files will be processed by linters (ex: `.*src/.*`)                                                                                                                                |
 | **GITHUB_ACTIONS_CONFIG_FILE**     | `actionlint.yml`                | Filename for [Actionlint configuration](https://github.com/rhysd/actionlint/blob/main/docs/config.md) (ex: `actionlint.yml`)                                                                                         |
 | **GITHUB_DOMAIN**                  | `github.com`                    | Specify a custom Github domain in case Github Enterprise is used: e.g. `github.myenterprise.com`                                                                                                                     |
-| **GITHUB_CUSTOM_API_URL**          | `api.github.com`                | Specify a custom Github API URL in case Github Enterprise is used: e.g. `https://github.myenterprise.com/api/v3/`                                                                                                    |
+| **GITHUB_CUSTOM_API_URL**          | `https://api.github.com`        | Specify a custom Github API URL in case Github Enterprise is used: e.g. `https://github.myenterprise.com/api/v3`                                                                                                     |                                                                                         |
 | **GITLEAKS_CONFIG_FILE**           | `.gitleaks.toml`                | Filename for [GitLeaks configuration](https://github.com/zricethezav/gitleaks#configuration) (ex: `.geatleaks.toml`)                                                                                                 |
 | **IGNORE_GENERATED_FILES**         | `false`                         | If set to `true`, super-linter will ignore all the files with `@generated` marker but without `@not-generated` marker.                                                                                               |
 | **IGNORE_GITIGNORED_FILES**        | `false`                         | If set to `true`, super-linter will ignore all the files that are ignored by Git.                                                                                                                                    |