Improve a code snippet on README

[seratch]

Summary

This pull request improves the RTM code snippet on README. Currently, the code fails every time a bot message is posted with KeyError as below:

(omitted)
  File "app.py", line 11, in say_hello
    if data and 'Hello' in data['text']:
KeyError: 'text'

Requirements (place an x in each [ ])

• 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.

[seratch]

Thanks for checking this!