Update classes.md (#1065)
Update classes.md (#1065)

It does not seem that adding the "open" annotation on members of a final class is prohibited as it compiles and runs fine.
IntelliJ comment on this:  'open' has no effect in a final class.
hltj/kotlin-web-site-cn,hltj/kotlin-web-site-cn,hltj/kotlin-web-site-cn,hltj/kotlin-web-site-cn,JetBrains/kotlin-web-site,JetBrains/kotlin-web-site,hltj/kotlin-web-site-cn,JetBrains/kotlin-web-site,JetBrains/kotlin-web-site,JetBrains/kotlin-web-site,JetBrains/kotlin-web-site,hltj/kotlin-web-site-cn