Add missing bash line continuation to a "WireMock in Docker" example

[rcw5]

The documentation is missing a line continuation from the example "Passing command-line arguments as environment variable"

This PR adds the missing \ character so the command can be copy-pasted into a terminal.

References

Original PR for the documentation: https://github.com/wiremock/wiremock.org/pull/216

Submitter checklist

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

Details: Contributor Guide