Merge branch 'fix_docs_warning' into 'NIFTy_7'

Fix minor docs build warning

See merge request !636
14 jobs for NIFTy_8 in 13 minutes and 43 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Static Checks
passed check_no_asserts #1342136

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  Build Docker
passed build_docker_from_cache #1342137

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  Test
passed test_mpi #1342139

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passed test_serial #1342138

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85.0%
 
  Demo Runs
passed run_bernoulli #1342147

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passed run_curve_fitting #1342148

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passed run_getting_density #1342146

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passed run_getting_started_1 #1342142

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passed run_getting_started_2 #1342143

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passed run_getting_started_3 #1342144

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passed run_getting_started_mf #1342145

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passed run_ipynb0 #1342140

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passed run_ipynb1 #1342141

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passed run_visual_vi #1342149

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