Add missing JSDoc `@param` in variable.js
Add missing JSDoc `@param` in variable.js

While looking into a bug, I noticed that there was a missing `@param`
here.

AlexKVal/eslint-plugin-react,petersendidit/eslint-plugin-react,mathieumg/eslint-plugin-react,yannickcr/eslint-plugin-react,pwmckenna/eslint-plugin-react,lencioni/eslint-plugin-react