Update tutorial.md

Only TWO types of variables are shown. But perhaps there is a missing bullet?

examples/official-site/your-first-sql-website/tutorial.md

@@ -142,7 +142,7 @@ It uses a `WHERE` clause to make sure that the `INSERT` statement is only execut
 The `:Username` parameter is set to `NULL` when you initially load the page, and then SQLPage automatically sets it to the value 
 from the text field when the user submits the form.
 
-There are three types of parameters you can use in your SQL queries:
+There are two types of parameters you can use in your SQL queries:
  - `:ParameterName` is a [POST](https://en.wikipedia.org/wiki/POST_(HTTP)) parameter. It is set to the value of the field with the corresponding `name` in a form. If no form was submitted, it is set to `NULL`.
  - `$ParameterName` works the same as `:ParameterName`, but it can also be set through a [query parameter](https://en.wikipedia.org/wiki/Query_string) in the URL.
     If you add `?x=1&y=2` to the end of the URL of your page, `$x` will be set to the string `'1'` and `$y` will be set to the string `'2'`.
@@ -202,4 +202,4 @@ To access your website, enter the address of your VPS in your address bar, follo
 
 For production use, you should:
  - run SQLPage as a service, using a tool like [docker](https://docs.docker.com/engine/reference/run/) or [systemd](https://linuxhandbook.com/create-systemd-services/).
- - [use a reverse proxy like *nginx*](./nginx.sql) to improve security and performance, or to configure multiple websites on the same server.
+ - [use a reverse proxy like *nginx*](./nginx.sql) to improve security and performance, or to configure multiple websites on the same server.