Make javadoc <p> style guide compliant

This led to confusion for an open source contributor about how to format
code. We don't want to be like, "do as I say, not as I do."

https://google.github.io/styleguide/javaguide.html#s7.1.2-javadoc-paragraphs
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=122589700

java/google/registry/model/domain/ReferenceUnion.java

@@ -28,8 +28,8 @@ import javax.xml.bind.annotation.adapters.XmlAdapter;
 /**
  * Legacy shell of a "union" type to represent referenced objects as either a foreign key or as a
  * link to another object in the datastore. In its current form it merely wraps a {@link Ref}.
- * <p>
- * This type always marshals as the "foreign key". We no longer use this type for unmarshalling.
+ *
+ * <p>This type always marshals as the "foreign key". We no longer use this type for unmarshalling.
  *
  * @param <T> the type being referenced
  */