MINOR: [C#] Fixing example to use WriteEndAsync instead of WriteFoote…

…rAsync

WriteFooterAsync is private, so the example doesn't compile.
This method was probably public in an earlier version of the library.
WriteEndAsync seems to be the proper replacement.

Closes #10399 from royalstream/patch-1

Authored-by: Steven Burns <[email protected]>
Signed-off-by: Eric Erhardt <[email protected]>

csharp/examples/FluentBuilderExample/Program.cs

@@ -51,7 +51,7 @@ public static async Task Main(string[] args)
             using (var writer = new ArrowFileWriter(stream, recordBatch.Schema))
             {
                 await writer.WriteRecordBatchAsync(recordBatch);
-                await writer.WriteFooterAsync();
+                await writer.WriteEndAsync();
             }
 
             Console.WriteLine("Done");