docs: fix typos in UPGRADING.md

[reichenbch]

dialogvlow_v2 to dialogflow_v2

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes #<issue_number_goes_here> 🦕

[busunkim96]

Thank you!

[busunkim96]

@reichenbch Could you review https://github.com/googleapis/python-dialogflow/pull/472/checks?check_run_id=5090689460 and sign the Contributor License Agreement?

[reichenbch]

@reichenbch Could you review https://github.com/googleapis/python-dialogflow/pull/472/checks?check_run_id=5090689460 and sign the Contributor License Agreement?

@busunkim96 Signed the CLA.

[dandhlee]

Thank you! Confirmed the CLA has been signed, updating commit header to align with conventional commit style.