[R client] Use %f instead of %d in sprintf format for numerics

[bmordue]

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, ./bin/openapi3/{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\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
• Filed the PR against the correct branch: master, 4.1.x, 5.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

This is a small fix for JSON serialization using sprintf of 'numeric' types in R that is already in swagger-codegen:
Relevant issue: swagger-api/swagger-codegen#8531

cc @andypetrella

Note: running the sample generation script ./bin/openapi3/r-petstore.sh has added new files.

[bmordue]

Hmm. This PR passed 3/4 checks: Shippable failed with some kind of build error unrelated to this change.
I force-pushed the same file changes, but with an edited commit message, just to retrigger the build checks. (It looks like I can't manually retrigger a failed job on Shippable or CircleCI without repo write access).
Second time, 3/4 checks passed: CircleCI failed this time, again I believe unrelated to my change.

So all 4 checks have passed for this PR, but not at the same time.

[bmordue]

@wing328 any thoughts about the failing build (timed out)?

[wing328]

@bmordue there's a merged PR improving R's model: #3099

Please check it out.

             {{#isNumeric}}[%d]{{/isNumeric}}{{^isNumeric}}[%s]{{/isNumeric}}{{#hasMore}},{{/hasMore}}

I think we need to keep %d for integer and long using for example {{#isInteger}} ... {{/isInteger}}

[bmordue]

@wing328 That's great, thanks for pointing that out.