adding missing parentheses to fix input width calculation

The expression for `A`, the amount of horizontal space available to the inner box in the diagram below, is `A = W - (L2 - L1)`. Omitting the parentheses is an error.

```
               [          [                        ⟧
                          -------------------------- A
               ------------------------------------- W
--------------- L1
-------------------------- L2
               ----------- (L2 - L1)
```