Fix example in documentation

[MarkMarkyMarkus]

Motivation:

There is an example of json builder in the documentation. It doesn't even compile, there should be obj() wrapper.

  @Test
  fun testJsonBlock() {
    val result = json {
      obj(
        "firstName" to "Dale",
        "lastName" to "Cooper",
        "age" to 64,
        "names" to array("Dale", "Bartholomew")
      )
    }

    assertEquals(
      """{"firstName":"Dale","lastName":"Cooper","age":64,"names":["Dale","Bartholomew"]}""",
      result.toString()
    )
  }

[vietj]

thanks