Bad .vtu file written by poisson.jl

I started the first tutorial, running the commands
```
using Gridap
model = DiscreteModelFromFile("../models/model.json")
writevtk(model,"model")
```
The files `model_*.vtu` were written as expected, however when I tried opening `model_2.vtu` in paraview, I got the following error messages.

> ERROR: In vtkXMLParser.cxx, line 364
> vtkXMLDataParser (0x55dab2e50fa0): Error parsing XML in stream at line 31, column 1, byte index 2075: not well-formed (invalid token)

> ERROR: In vtkXMLReader.cxx, line 576
> vtkXMLUnstructuredGridReader (0x55dab2c58f20): Error parsing input file. ReadXMLInformation aborting.

My Julia `versioninfo()` output is

> Julia Version 1.10.4
> Commit 48d4fd48430 (2024-06-04 10:41 UTC)
> Build Info:
> Official https://julialang.org/ release
> Platform Info:
> OS: Linux (x86_64-linux-gnu)
> CPU: 16 × AMD Ryzen 7 3700X 8-Core Processor
> WORD_SIZE: 64
> LIBM: libopenlibm
> LLVM: libLLVM-15.0.7 (ORCJIT, znver2)
> Threads: 1 default, 0 interactive, 1 GC (on 16 virtual cores)

and I am using ParaView 5.12.0 packaged by Fedora 40.

Riproduci i comandi del tutorial usando ../models/model.json e ispeziona model_2.vtu intorno alla riga 31, dove ParaView segnala che l'XML non è ben formato. Confronta il file generato con gli altri file model_*.vtu e verifica che l'output corretto si apra correttamente in ParaView 5.12.0.

Scritto dal modello di indicizzazione a partire dal testo della issue.