Fix syntax highlighting and links

Author: michel-kraemer

vertx-auth-common/src/main/asciidoc/index.adoc

@@ -62,6 +62,7 @@ To authenticate a user you use {@link io.vertx.ext.auth.authentication.Authentic
 The first argument is a Credentials object which contains authentication information.
 What this actually contains depends on the specific implementation; for a simple username/password based authentication it might contain something like:
 
+[source,json]
 ----
 {
   "username": "tim"
@@ -219,8 +220,8 @@ PEM files are common and easy to use but are not password protected, so private
 
 JWKs are a standard used by OpenID connect and JWT providers.
 They represent a key as a JSON object.
-Usually these JSON documents are provided by an identity provider server like Google, Microsoft, etc... but you can also generate your own keys using the online application <a href="https://mkjwk.org/">https://mkjwk.org</a>.
-For an offline experience there is also the tool: <a href="https://connect2id.com/products/nimbus-jose-jwt/generator">https://connect2id.com/products/nimbus-jose-jwt/generator</a>.
+Usually these JSON documents are provided by an identity provider server like Google, Microsoft, etc... but you can also generate your own keys using the online application https://mkjwk.org.
+For an offline experience there is also the tool: https://connect2id.com/products/nimbus-jose-jwt/generator.
 
 == Chaining authentication providers