Add missing keyword to glif neuron model documentation #4520

Job	Run time
cppcheck	3m 43s
vale	34s
black	25s
clang-format	17s
copyright_headers	17s
flake8	21s
forbidden_types	14s
isort	10s
lychee_IGNORED	25s
mypy	41s
pydocstyle	24s
pylint	33s
rstcheck	20s
unused_names	16s
sphinx-rtd	6m 56s
sphinx-conda	0s
build_linux (ubuntu-20.04, gcc, boost, optimize, warning)	12m 26s
build_macos (macos-latest, clang, openmp, mpi, python, gsl, ltdl, boost, hdf5, optimize, warning)	25m 0s
build_linux (ubuntu-20.04, gcc, openmp, python, gsl, ltdl, boost, optimize, warning)	16m 20s
build_linux (ubuntu-20.04, gcc, mpi, python, gsl, ltdl, boost, optimize, warning)	18m 27s
build_linux (ubuntu-20.04, gcc, openmp, mpi, python, gsl, ltdl, boost, hdf5, sionlib, libneurosim...	23m 24s
	1h 51m 13s

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