Added example rollback scripts (#2367)

When running the examples with the example-changelog.sql you cannot rollback your changes.
This enhancement allows you to see the example scripts you would use to rollback your changes and allows you to use the rollback commands with this changelog. Also helps the init project

liquibase-core/src/main/resources/liquibase/examples/sql/example-changelog.sql

@@ -8,6 +8,7 @@ create table person (
     address2 varchar(50),
     city varchar(30)
 )
+--rollback DROP TABLE person;
 
 --changeset your.name:2
 create table company (
@@ -17,7 +18,9 @@ create table company (
     address2 varchar(50),
     city varchar(30)
 )
+--rollback DROP TABLE company;
 
 --changeset other.dev:3
 alter table person add column country varchar(2)
+--rollback ALTER TABLE person DROP COLUMN country;