Fix typo in OneOfBuilder documentation

[elfenlaid]

Hello 👋 Hope all is well

The changes fix a small type for OneOfBuilder.swift documentation

I've took the example from its tests

final class OneOfBuilderTests: XCTestCase {
  func testBuildArray() {
    enum Role: String, CaseIterable, Equatable {
      case admin
      case guest
      case member
    }

    let parser = OneOf {
      for role in Role.allCases {
        role.rawValue.map { role }
      }
    }
    //...

[mbrandonw]

Good catch, thanks!