square-grid: fix package description mentioning wrong parameters in doc

[raphael-leger]

Creates a square grid from a bounding box, Feature or FeatureCollection.
=> Creates a square grid from a bounding box.

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Closes #1830

• Use a meaningful title for the pull request. Include the name of the package modified.
• Have read How To Contribute.
• Run npm test at the sub modules where changes have occurred.
• Run npm run lint to ensure code style at the turf module level.

[rowanwins]

Looks good - thanks @raphael-leger !