Update index.md to beta-4

Author: slachiewicz

src/site/markdown/index.md

@@ -4,5 +4,5 @@ A collection of various utility classes to ease working with XML.
 
 This library consists of XML classes (`org.codehaus.plexus.util.xml` and `org.codehaus.plexus.util.xml.pull`):
 1. that have been extracted from [`plexus-utils`](../plexus-utils/) 4,
-2. then updated to use [`maven-xml-api`](https://maven.apache.org/ref/4.0.0-alpha-7/api/maven-api-xml/)/[`maven-xml-impl`](https://maven.apache.org/ref/4.0.0-alpha-7/maven-xml-impl/index.html)
+2. then updated to use [`maven-xml-api`](https://maven.apache.org/ref/4.0.0-beta-4/api/maven-api-xml/)/[`maven-xml-impl`](https://maven.apache.org/ref/4.0.0-beta-4/maven-xml-impl/index.html)