Update 10.20 to match book and update test as well

src/Chapter10.Tests/Listing10.20.Tests.cs

@@ -47,7 +47,7 @@ private static void DeleteTempFile()
         public void Create_GivenNewDocument_FileGetsCreated()
         {
             TemporaryFileStream fileStream = new TemporaryFileStream(TempFileName);
-            Assert.IsTrue(File.Exists(fileStream.File.FullName));
+            Assert.IsTrue(File.Exists(fileStream.File?.FullName!));
         }
 
         [TestMethod]

src/Chapter10/Listing10.20.DefiningAFinalizer.cs

@@ -34,13 +34,22 @@ public TemporaryFileStream()
             }
         }
 
-        public FileStream Stream { get; }
-        public FileInfo File { get; }
+        public FileStream Stream { get; private set; }
+        public FileInfo File { get; private set; }
 
         public void Close()
         {
             Stream?.Dispose();
-            File?.Delete();
+            try
+            {
+                File?.Delete();
+            }
+            catch(IOException exception)
+            {
+                Console.WriteLine(exception);
+            }
+            Stream = null;
+            File = null;
         }
     }
 }