Update README.md

[scubafly]

When I run: python3 pymodoro.py 25 I got the following error: Traceback (most recent call last): File "~/projects/pymodoro/pymodoro.py", line 5, in import tkinter as tk File "/opt/homebrew/Cellar/[email protected]/3.12.3/Frameworks/Python.framework/Versions/3.12/lib/python3.12/tkinter/init.py", line 38, in import _tkinter # If this fails your Python may not be configured for Tk ^^^^^^^^^^^^^^^ ModuleNotFoundError: No module named '_tkinter'

Fixed by installing python-tk. Updated readme how to do this.