Merge branch 'doc_fixups' into 'NIFTy_7'

Fix warning in doc generation

See merge request !554
12 jobs for NIFTy_7 in 25 minutes and 59 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build Docker
passed #1107390
build_docker_from_scratch

00:12:22

 
  Test
passed #1107392
test_mpi

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passed #1107391
test_serial

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86.0%
 
  Demo Runs
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run_bernoulli

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passed #1107400
run_curve_fitting

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passed #1107395
run_getting_started_1

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passed #1107396
run_getting_started_2

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passed #1107397
run_getting_started_3

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passed #1107398
run_getting_started_mf

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passed #1107393
run_ipynb0

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passed #1107394
run_ipynb1

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passed #1107401
run_visual_mgvi

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