chore: update badge

README.md

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 # Beancount N26 Importer
 
 [![image](https://github.com/siddhantgoel/beancount-n26/workflows/beancount-n26/badge.svg)](https://github.com/siddhantgoel/beancount-n26/workflows/beancount-n26/badge.svg)
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 [![image](https://img.shields.io/pypi/v/beancount-n26.svg)](https://pypi.python.org/pypi/beancount-n26)
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 [![image](https://img.shields.io/pypi/pyversions/beancount-n26.svg)](https://pypi.python.org/pypi/beancount-n26)
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-[![image](https://img.shields.io/badge/code%20style-black-000000.svg)](https://github.com/psf/black)
+[![Ruff](https://img.shields.io/endpoint?url=https://raw.githubusercontent.com/astral-sh/ruff/main/assets/badge/v2.json)](https://github.com/astral-sh/ruff)
 
 `beancount-n26` provides a [Beancount] Importer for converting CSV exports of
 [N26] account summaries to the Beancount format.