Adjusted Passenger schema in KProperties API Docs

[holgerbrandl]

Adjusted Passenger schema to reflect nullability of age attribute. With the fix, we can also use filter: .filter{ it[Passenger::age]!=null}

[koperagen]

Hi! It's a bit trickier :) This sample is copy-pasted by korro task from

dataframe/tests/src/test/kotlin/org/jetbrains/kotlinx/dataframe/samples/api/ApiLevels.kt

Line 82 in d8c6b2f

val age: Int,

It should be changed there as well

[Jolanrensen]

There are more things that aren't in the actual titanic.csv file:
The delimiter is wrong, survived is 1/0, not a boolean, there are more attributes etc. So I think it's intentional age is not Int?. It's just an example.
@koperagen should these docs actually work with our titanic.csv?