Fix javadoc build errors when using jdk8

Building with java 8 created errors like this:

    src/main/java/net/logstash/logging/log4j2/core/layout/LogStashJSONLayout.java:123: error: header used out of sequence: <H4>
     * <h4>Pretty vs. compact XML</h4>
    src/main/java/net/logstash/logging/log4j2/core/layout/LogStashJSONLayout.java:91: error: bad use of '>'
     *                "message" => "Going on right here2",
                                 ^
    src/main/java/net/logstash/logging/log4j2/core/layout/LogStashJSONLayout.java:571: error: self-closing element not allowed
         * <p/>
           ^