Update DataClasses.kt

[FreekDB]

As a Kotlin newbie and Java developer, I was assuming the data annotation would start with an at sign (@data). Perhaps some additional information would be helpful, like for example: (this will look like this: "data class Person").

[yole]

The correct fix is to change "annotation" to "modifier" in the text. "data" used to be an annotation in pre-release versions of Kotlin, but now it's a modifier.

[FreekDB]

Dmitry Jemerov: "The correct fix is to change "annotation" to "modifier" in the text. "data" used to be an annotation in pre-release versions of Kotlin, but now it's a modifier."

[svtk]

@FreekDB, thanks for noticing. I've updated the text.