[R] Fix escaping in templates

[avandecreme]

This spec generates failing code (error during R CMD check) because some fields have an underscore prefix.
This PR adds some backticks where necessary.

@Ramanth @saigiridhar21

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[wing328]

Thanks for the PR but your commit (as shown in the Commits tab) is not linked to your Github account, which means this PR won't count as your contribution in https://github.com/OpenAPITools/openapi-generator/graphs/contributors.

Let me know if you need help fixing it.

Ref: https://github.com/OpenAPITools/openapi-generator/wiki/FAQ#how-can-i-update-commits-that-are-not-linked-to-my-github-account

[avandecreme]

Thanks, fixed and rebased

[wing328]

@avandecreme thanks for the PR.

Tested locally to confirm the issue and the fix.