issue 8481 -- added proper syntax for Swift4 with PromiseKit6

[dgaedcke]

PR checklist

• [ x] Read the contribution guidelines.
• [ ] Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• [ ] Filed the PR against the correct branch: 3.0.0 branch for changes related to OpenAPI spec 3.0. Default: master.
• [ ] Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

PromiseKit6 has introduced breaking syntax changes and I've modified the template accordingly. I'm brand new at all this and have no clue how to copy the technical committee, etc, but I did regenerate and build correctly compiling swift4/pmk6 code.