Update README to beta-4

Author: slachiewicz

README.md

@@ -12,7 +12,7 @@ This library consists of XML classes (`org.codehaus.plexus.util.xml` and `org.co
    this step is released as `plexus-xml` 3, maintained in [3.x branch](tree/3.x)\
    [![Maven Central](https://img.shields.io/maven-central/v/org.codehaus.plexus/plexus-xml.svg?label=Maven%20Central&versionPrefix=3.)](https://search.maven.org/artifact/org.codehaus.plexus/plexus-xml),
 
-2. then updated to use Maven 4-specific [`maven-xml-api`](https://github.com/apache/maven/tree/maven-4.0.0-alpha-7/api/maven-api-xml)/[`maven-xml-impl`](https://github.com/apache/maven/tree/maven-4.0.0-alpha-7/maven-xml-impl):\
+2. then updated to use Maven 4-specific [`maven-xml-api`](https://github.com/apache/maven/tree/maven-4.0.0-beta-4/api/maven-api-xml)/[`maven-xml-impl`](https://github.com/apache/maven/tree/maven-4.0.0-beta-4/maven-xml-impl):\
    this is the `master` branch from which `plexus-xml` 4 is released\
    Version 4.x requires Java 17 (like Maven 4)
    [![Maven Central](https://img.shields.io/maven-central/v/org.codehaus.plexus/plexus-xml.svg?label=Maven%20Central)](https://search.maven.org/artifact/org.codehaus.plexus/plexus-xml)