docs: use introduction in the side bar (#235)

Author: timdeschryver

docs/native-testing-library/intro.md

@@ -1,7 +1,7 @@
 ---
 id: intro
 title: Introduction
-sidebar_label: Intro
+sidebar_label: Introduction
 ---
 
 Native Testing Library is a testing library for **React Native** inspired by

docs/vue-testing-library/intro.md

@@ -1,6 +1,7 @@
 ---
 id: intro
 title: Intro
+sidebar_label: Introduction
 ---
 
 Vue Testing Library builds on top of `DOM Testing Library` by adding APIs for