Typo in onChange example

[mattcobb]

What are you trying to achieve or the steps to reproduce?

Fixing a typo in the onChange() example comments in packages/botkit/src/conversation.ts

     * convo.onChange('name', async(response, convo, bot) {

should be

     * convo.onChange('name', async(response, convo, bot) => {

What was the result you received?

When the example is copied into a program, it has a syntax error.

What did you expect?

Syntactically correct example

Context:

• Botkit version: 4.5
• Messaging Platform: msteams
• Node version: 10.15
• Os: Mac & Linux
• Any other relevant information:
  I made this fix local and I'll create a PR for it