[.NET] Use collection expressions in docs

As of C# 12 we can now use collection expressions to reduce some boilerplate when initializing collections.

doc/classes/HTTPClient.xml

@@ -156,7 +156,7 @@
 				[csharp]
 				var fields = new Godot.Collections.Dictionary { { "username", "user" }, { "password", "pass" } };
 				string queryString = new HttpClient().QueryStringFromDict(fields);
-				string[] headers = { "Content-Type: application/x-www-form-urlencoded", $"Content-Length: {queryString.Length}" };
+				string[] headers = ["Content-Type: application/x-www-form-urlencoded", $"Content-Length: {queryString.Length}"];
 				var result = new HttpClient().Request(HttpClient.Method.Post, "index.php", headers, queryString);
 				[/csharp]
 				[/codeblocks]