Update CHANGELOG.md

Author: pancetta

CHANGELOG.md

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--   April 11, 2025: Version 5.6 adds a framework for MPI parallel IO, developed by [\@tlunet](https://github.com/tlunet), making it easy to visualize the data obtained with pySDC on HPC machines in software such as ParaView.
+-   April 11, 2025: Version 5.6 adds a framework for MPI-parallel I/O, developed by [\@tlunet](https://github.com/tlunet), making it easier to visualize the data obtained with pySDC on HPC machines in software such as ParaView.
     Also, pySDC is now compatible with the finite element library [Firedrake](https://github.com/firedrakeproject/firedrake) and the geophysical fluid dynamics library
     [Gusto](https://github.com/firedrakeproject/gusto), thanks to [\@jshipton](https://github.com/jshipton) and [\@brownbaerchen](https://github.com/brownbaerchen).
     The former allows to setup PDEs with finite element discretizations in pySDC and then solve in time with SDC and PFASST, while the latter allows to setup a geophysical fluid dynamics problem and then use pySDC with any SDC setup as a timestepper in Gusto.