Update Gruntfile.js

[liliyao2022]

updated regular functions to arrow function.

Fixes #0000 (<=== Add issue number here)

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[cesswairimu]

Hi @liliyao2022, it seems the changes you have already been made in the file, could be someone was already working on them when you started. In future please check if someone is working on an issue before you get started. Closing this, thanks