Update readme with ubuntu 23 finish recording workaround

[jasonsoooz]

What's in this PR?

• update readme with workaround for ubuntu 23 finish recording issue (raised in issue 85)

Reasoning

• ubuntu 23 has issues stopping recording as menu is grey and not selectable and keyboard shortcut doesn't work consistently -> kazam not usable
• there is a simple workaround by installing python packages which allows the finish recording shortcut to work well -> kazam usable again
• mention this in readme for ubuntu 23 users who might run into this issue