supplementaryViewProvider optional type documentation

Author: FelixHerrmann

README.md

@@ -114,7 +114,7 @@ tableView.register(CustomCell.self, forCellReuseIdentifier: "customCell")
 In order to use custom header or footer views, the `supplementaryViewProvider` property must be overritten.
 
 ```swift 
-override var supplementaryViewProvider: UICollectionViewDiffableDataSource<Section, Item>.SupplementaryViewProvider {
+override var supplementaryViewProvider: UICollectionViewDiffableDataSource<Section, Item>.SupplementaryViewProvider? {
     return { (collectionView, kind, indexPath) in
         let headerView = collectionView.dequeueReusableSupplementaryView(ofKind: kind, withReuseIdentifier: "customHeader", for: indexPath) as? CustomHeader
         switch self.dataSource.snapshot().sectionIdentifiers[indexPath.section] {

Sources/FHDiffableViewControllers/FHDiffableCollectionViewController.swift

@@ -121,7 +121,7 @@ open class FHDiffableCollectionViewController<SectionIdentifierType, ItemIdentif
     ///
     /// The default implementation just shows an empty section header.
     ///
-    ///     override var supplementaryViewProvider: UICollectionViewDiffableDataSource<SectionIdentifierType, ItemIdentifierType>.SupplementaryViewProvider {
+    ///     override var supplementaryViewProvider: UICollectionViewDiffableDataSource<SectionIdentifierType, ItemIdentifierType>.SupplementaryViewProvider? {
     ///         return { (collectionView, kind, indexPath) in
     ///             let supplementaryView = collectionView.dequeueReusableSupplementaryView(ofKind: kind, withReuseIdentifier: /*your identifier*/, for: indexPath) as? CustomReuseableView
     ///             /*customize your supplementary view here*/