Fix "Using a custom configuration" link in README

Fix the "Using a custom configuration" link the README. The actual ID of the header on the page is "using-a-custom-configuration-file", so I updated both the link and the link test to match. (as far as I can tell all other links in the README work as expected)

README.md

@@ -98,7 +98,7 @@ Use the `config-file` parameter of the `init` action to enable the configuration
     config-file: ./.github/codeql/codeql-config.yml
 ```
 
-The configuration file must be located within the local repository. For information on how to write a configuration file, see "[Using a custom configuration](https://help.github.com/en/github/finding-security-vulnerabilities-and-errors-in-your-code/configuring-code-scanning#using-a-custom-configuration)."
+The configuration file must be located within the local repository. For information on how to write a configuration file, see "[Using a custom configuration file](https://help.github.com/en/github/finding-security-vulnerabilities-and-errors-in-your-code/configuring-code-scanning#using-a-custom-configuration-file)."
 
 If you only want to customise the queries used, you can specify them in your workflow instead of creating a config file, using the `queries` property of the `init` action: