slight UI tweak

docs/index.md

@@ -4,8 +4,8 @@
 ![banner](img/banner-biometric-auth.svg)
 
 <center style="margin:-1rem 0 3rem;">
-<a href="/demos/basic.html" class="btn btn-primary btn-lg mx-3 px-5">&#x27A4; Demo</a>
-<a href="/demos/playground.html" class="btn btn-primary btn-lg mx-3 px-4">&#x27A4; Playground</a>
+<a href="/demos/basic.html" class="btn btn-primary btn-lg m-3 px-5">&#x27A4; Demo</a>
+<a href="/demos/playground.html" class="btn btn-primary btn-lg m-3 px-4">&#x27A4; Playground</a>
 </center>
 
 > This library greatly simplifies the usage of **passkeys** by invoking the [WebAuthn protocol](https://w3c.github.io/webauthn/) more conveniently. It is [open source](https://github.com/passwordless-id/webauthn), opinionated, dependency-free and minimalistic (20kb only, from which 13kb is the list of authenticator AAGUIDs/names).