Fix intercharacter gap calculation in Code39View

Modify the calculation of intercharacterGapCount in Code39View.swift:
- Subtract 1 from the result of (barcode.count % 9)

Author: jiahan-wu

Sources/Code39/Code39View.swift

@@ -17,7 +17,7 @@ public struct Code39View: View {
 
     public var body: some View {
         Canvas { context, size in
-            let intercharacterGapCount = barcode.count % 9
+            let intercharacterGapCount = (barcode.count % 9) - 1
             let narrowBarWidth = size.width / CGFloat(((((barcode.count - intercharacterGapCount) / 9) * 15) + intercharacterGapCount))
             var currentX: CGFloat = 0
             for (index, barWidth) in barcode.enumerated() {