Correct javadoc for GoodbyeEvent

[KingBuzzer]

Correct javadoc for the GoodbyeEvent

Category (place an x in each of the [ ])

• bolt (Bolt for Java)
• bolt-{sub modules} (Bolt for Java - optional modules)
• slack-api-client (Slack API Clients)
• slack-api-model (Slack API Data Models)
• slack-api-*-kotlin-extension (Kotlin Extensions for Slack API Clients)
• slack-app-backend (The primitive layer of Bolt for Java)

Requirements

Please read the Contributing guidelines and Code of Conduct before creating this issue or pull request. By submitting, you are agreeing to the those rules.

[seratch]

LGTM - thanks!

[seratch]

Some tests are failing but this change is unrelated. It's safe enough to merge this (I will look at the occasional test failure)

Running test_locally.SocketModeAppTest
Tests run: 2, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 5.985 sec <<< FAILURE!
payloadHandling(test_locally.SocketModeAppTest)  Time elapsed: 2.181 sec  <<< FAILURE!
java.lang.AssertionError
	at org.junit.Assert.fail(Assert.java:87)
	at org.junit.Assert.assertTrue(Assert.java:42)
	at org.junit.Assert.assertTrue(Assert.java:53)
	at test_locally.SocketModeAppTest.payloadHandling(SocketModeAppTest.java:73)