MINOR: change updateBrokerConfig and updateDefaultConfig error message

[TaiJuWu]

The original error show java.util.Collections$3@af78c87, after change
it show the [config.key.3, config.key.1, config.key.2]

Example:

  @Test
  def testPropertiesContent(): Unit = {
    val props = new Properties()
    props.put("config.key.1", "value1")
    props.put("config.key.2", "value2")
    props.put("config.key.3", "value3")

    System.err.println("\nProperties content:") System.err.println(s"
props = $props")        // output: props = {config.key.3=value3,
config.key.2=value2, config.key.1=value1}

    System.err.println(s"\n  props.keySet() = ${props.keySet()}")  //
output: props.keySet() = [config.key.3, config.key.2, config.key.1]

    System.err.println(s"\n  Set from keys = ${props.keys()}")
// output: Set from keySet = java.util.Collections$3@af78c87
  }

Reviewers: Lan Ding isDing_L@163.com, Ken Huang s7133700@gmail.com,
PoAn Yang payang@apache.org