fix: correct variable name in JS example

[MiguelArgentina]

Motivation / Background

This Pull Request has been created because an example of Working with JavaScript in Rails uses a variable name that, in my opinion, is not the one available in the scope where it is being used.

Detail

This Pull Request changes the name of the variable in line 47 of the file rails/guides/source/working_with_javascript_in_rails.md

Additional information

N/A

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.

[jonathanhefner]

I can merge this fix, but it will not show up at https://guides.rubyonrails.org/v6.1/working_with_javascript_in_rails.html until a 6.1.8 release is made. And it is unlikely that 6.1.8 will ever be released because the 6.1 series is only receiving security patches, per our maintenance policy. In other words, any future 6.1 releases will probably be 6.1.7.x, not 6.1.x.

Still, thank you for the pull request, @MiguelArgentina! 🎉