fix stub mapping http endpoint

[siph]

This was seemingly changed at some point (based on wiremock/wiremock#253 (comment)) and I'm guessing this got missed because it is corrected elsewhere in the documentation.

References

https://wiremock.org/docs/stubbing/#basic-stubbing:

You can configure stubs using JSON configuration files or code:

1. Via a `.json` file under the `mappings` directory
2. Via a POST request to
`http://<host>:<port>/__admin/mappings` with the JSON as a body
3. From code using one of the SDKs

Submitter checklist

• The PR request is well described and justified, including the body and the references
• The PR title represents the desired changelog entry
• If the change against WireMock 2 functionality (incompatible with WireMock 3),
  it is submitted against the 2.x branch
• The repository's code style is followed (see the contributing guide)

Details: Contributor Guide

[leeturner]

Great spot. Many thanks for your contribution