Add a ? in the Children section for optional collections

Potentially, this is why I didn’t think about the fact that syntax collections can be optional.

Author: ahoppen

CodeGeneration/Sources/SyntaxSupport/GrammarGenerator.swift

@@ -37,7 +37,7 @@ struct GrammarGenerator {
       let choicesDescriptions = choices.map { grammar(for: $0) }
       return "(\(choicesDescriptions.joined(separator: " | ")))\(optionality)"
     case .collection(kind: let kind, _, _, _):
-      return "``\(kind.syntaxType)``"
+      return "``\(kind.syntaxType)``\(optionality)"
     case .token(let choices, _, _):
       if choices.count == 1 {
         return "\(grammar(for: choices.first!))\(optionality)"