Improve Kotlin example in manual

[lukaseder]

From:

DSL.using(
        properties.getProperty("db.url"),
        properties.getProperty("db.username"),
        properties.getProperty("db.password")
        
    // The below "use" method is an adapted version of the one in kotlin.io
    // See also: https://github.com/JetBrains/kotlin/pull/807
    ).use {
        val ctx = it
    ...

To:

DSL.using(
        properties.getProperty("db.url"),
        properties.getProperty("db.username"),
        properties.getProperty("db.password")
    ).use { ctx ->
    ...