toGridLengthHelper: fix double "fr" on "minmax"

[PedroHLC]

Expected behavior

fillPortion 128 |> minimum 1 should create a grid column with minmax(1px, 128fr).

Current behavior

CSS was generating something like:

.grid-rows-auto-cols-min1128fr-88fr-140fr-142fr-216fr-space-x-0px-space-y-0px {
  grid-template-columns: minmax(1px, 128frfr) 88fr 140fr 142fr 216fr;
  # etc etc etc
}

The frfr breaks the entire rule. And the browser goes wild.

Suggested fix

Remove the repetition, s/frfr/fr/.

There are no tests covering grid, so I also didn't add one for this issue.