Update ExcessiveImports example code for clarity

Since the default minimum is 30, I believe it would make more sense if the comment said "28" instead of "18".

pmd-java/src/main/resources/category/java/design.xml

@@ -632,7 +632,7 @@ user-specified threshold.
 <![CDATA[
 import blah.blah.Baz;
 import blah.blah.Bif;
-// 18 others from the same package elided
+// 28 others from the same package elided
 public class Foo {
     public void doWork() {}
 }