Fix assignment of new object instance to variable

[sneumann]

Fixes Error in foobar$fromJSONString(s) : object 'CategoryObject' not found
Discussion in #6520 (comment) ,
thanks @wing328 for encouraging.

PR checklist

• 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

(details of the change, additional tests that have been done, reference to the issue for tracking, etc)

[sneumann]

Hi, I did read CONTRIBUTING.md, I did not update petstore (I used the github online editor),
and I did not Cc the tech committee, since there is no member assigned to R.

[wing328]

@sneumann that's ok. Thanks for the PR, which looks good to me.

I'll merge it in the coming weekend.

[wing328]

@sneumann I also did a grep for "->" and confirm there's no other occurrence in the templates.

I've updated the R petstore sample via fb73b8c