chore(readme): add browserstack acknowledgement

Author: SapphoSys

README.md

@@ -204,7 +204,7 @@ Does the app work on other CalDAV servers or CalDAV-compatible clients that are
 Chiri is licensed under the [<span aria-hidden="true">&nearr;</span> zlib/libpng][repo-license] license.
 
 # Acknowledgements
-This project is tested with BrowserStack
+This project is tested with BrowserStack.
 
 [donate-kofi]: https://ko-fi.com/solelychloe
 [donate-liberapay]: https://liberapay.com/chloe