Update example code for subtype() middleware docs

[srajiang]

Description

The Filtering on message subtypes mention to use the subtype() middleware but doesn't explicitly call out that you need to import it from Bolt so you run into a subtype is not defined reference error.

We can have import in the code example use: https://api.slack.com/events/message/message_changed

Also the example currently uses bot_message but these don't currently show uyp with Granular Bot messages as per a outstanding bug.

What type of issue is this? (place an x in one of the [ ])

• bug
• enhancement (feature request)
• question
• documentation related
• example code related
• testing related
• discussion

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

• I've read and understood the Contributing guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've searched for any related issues and avoided creating a duplicate issue.