Change slash_commands to slash_command

https://github.com/johnagan/slack-payload/blob/master/index.js#L219

README.md

@@ -111,7 +111,7 @@ slack.on('event name', [... 'event name',] callback)
 slack.on('/test', message => { })
 
 // handle all slash commands
-slack.on('slash_commands', message => { })
+slack.on('slash_command', message => { })
 
 // handle the outgoing webhooks trigger word "googlebot"
 slack.on('googlebot', message => { })
@@ -219,4 +219,4 @@ let message = {
 }
 
 ws.send( JSON.stringify(message) )
-```
+```