Make readme batches consistent in color

Author: pnkraemer

README.md

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 # Matfree: Matrix-free linear algebra in JAX
 
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-[![image](https://img.shields.io/pypi/pyversions/matfree.svg)](https://pypi.python.org/pypi/matfree)
+[![CI](https://github.com/pnkraemer/matfree/workflows/ci/badge.svg)](https://github.com/pnkraemer/matfree/actions)
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 Randomised and deterministic matrix-free methods for trace estimation, functions of matrices, and matrix factorisations.
 Matfree builds on [JAX](https://jax.readthedocs.io/en/latest/).