Fix syntax

Author: ra1028

Sources/DiffableDataSourceSnapshot.swift

@@ -1,6 +1,6 @@
 /// A class for backporting `NSDiffableDataSourceSnapshot` introduced in iOS 13.0+, macOS 10.15+, tvOS 13.0+.
 /// Represents the mutable state of diffable data source of UI.
-public struct DiffableDataSourceSnapshot<SectionIdentifierType, ItemIdentifierType> where SectionIdentifierType : Hashable, ItemIdentifierType : Hashable {
+public struct DiffableDataSourceSnapshot<SectionIdentifierType: Hashable, ItemIdentifierType: Hashable> {
     internal var structure = SnapshotStructure<SectionIdentifierType, ItemIdentifierType>()
 
     /// Creates a new empty snapshot object.