llama.swiftui : add "Done" dismiss button to help view (#11998)

The commit updates the help view in the llama.swiftui example to use a
NavigationView and a Done button to dismiss the help view.

The motivation for this is that without this change there is now way to
dimiss the help view.

examples/llama.swiftui/llama.swiftui/UI/ContentView.swift

@@ -124,6 +124,7 @@ struct ContentView: View {
                     }
                 }
             }.sheet(isPresented: $showingHelp) {    // Sheet for help modal
+                NavigationView {
                     VStack(alignment: .leading) {
                         VStack(alignment: .leading) {
                             Text("1. Make sure the model is in GGUF Format")
@@ -133,6 +134,16 @@ struct ContentView: View {
                         }
                         Spacer()
                     }
+                    .navigationTitle("Help")
+                    .navigationBarTitleDisplayMode(.inline)
+                    .toolbar {
+                        ToolbarItem(placement: .navigationBarTrailing) {
+                            Button("Done") {
+                                showingHelp = false
+                            }
+                        }
+                    }
+                }
             }
         }
     }