Fix #9618
`@function grid-column` takes a list as argument and not a string.
/// Width of the column. Accepts multiple values:
/// - A percentage value will make the column that exact size.
/// - A single digit will make the column span that number of columns wide, taking into account the column count of the parent row.
-/// - A string of the format "x of y" will make a column that is *x* columns wide, assuming *y* total columns for the parent.
+/// - A list of the format "x of y" (without quotes) will make a column that is *x* columns wide, assuming *y* total columns for the parent.
///
/// @returns {Number} A calculated percentage value.
@function grid-column($columns) {