fix: rename variable name "student" to "city" in addOne method in CityStore

[tamim36]

Small changes in variable name in addOne method in CityStore. As per context the variable name Should be city instead of student.

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[tomalaforge]

@all-contributors add @tamim36 for bugs

[allcontributors]

@tomalaforge

I've put up a pull request to add @tamim36! 🎉