Janitor: fix missing period in docs

As reported in slint-ui#8448

Author: ogoffart

api/rs/slint/lib.rs

@@ -11,7 +11,7 @@
 This crate is the main entry point for embedding user interfaces designed with
 [Slint](https://slint.rs/) in Rust programs.
 */
-#![doc = i_slint_core_macros::slint_doc_str!("If you are new to Slint, start with the [Walk-through **tutorial**](slint:quickstart)")]
+#![doc = i_slint_core_macros::slint_doc_str!("If you are new to Slint, start with the [Walk-through **tutorial**](slint:quickstart).")]
 /*! If you are already familiar with Slint, the following topics provide related information.
 
 ## Topics