Link to abs rather than pdf on arXiv

[jlapeyre]

The following should link to the abstract rather than the pdf

forest-benchmarking/docs/examples/direct_fidelity_estimation.ipynb

Line 68 in 888068b

"The [average gate fidelity](https://arxiv.org/pdf/quant-ph/0205035.pdf) between an experimental process $\\mathcal E$ and the ideal (unitary) process $\\mathcal U$ is given by\n",