docs: add full stop to end of sentence (testing-library#931)

Author: HonkingGoose

docs/intro.mdx

@@ -38,7 +38,7 @@ is also a plugin to use testing-library queries for end-to-end tests in
 2.  Specific to a testing framework
 
 `DOM Testing Library` works with any environment that provides DOM APIs, such as
-Jest, Mocha + JSDOM, or a real browser
+Jest, Mocha + JSDOM, or a real browser.
 
 ### What you should avoid with Testing Library