You should never modify a <changeSet> that was already executed. Liquibase calculates checksums for all executed changeSets and stores them in the log. It will then recalculate that checksum, compare it to the stored ones and fail the next time you run it if the checksums differ.
What you need to do instead is to add another <changeSet> and put your new createTable element in it.
QuickStart is good readin’ but it is indeed quick 🙂 Check out the full manual, particularly its ChangeSet section.