DOC: Typos found by codespell

[DimitriPapadopoulos]

Running codespell manually for now, but could be automated as suggested in systemd/systemd#15436 if there is interest.

[codecov]

Codecov Report

Merging #1040 (b625d2e) into master (ea68c4e) will not change coverage.
The diff coverage is 50.00%.

@@           Coverage Diff           @@
##           master    #1040   +/-   ##
=======================================
  Coverage   92.26%   92.26%           
=======================================
  Files         100      100           
  Lines       12205    12205           
  Branches     2136     2136           
=======================================
  Hits        11261    11261           
  Misses        616      616           
  Partials      328      328           

Impacted Files	Coverage Δ
nibabel/affines.py	100.00% <ø> (ø)
nibabel/analyze.py	98.52% <ø> (ø)
nibabel/batteryrunners.py	95.83% <ø> (ø)
nibabel/casting.py	86.32% <ø> (ø)
nibabel/cmdline/diff.py	93.70% <0.00%> (ø)
nibabel/dataobj_images.py	98.36% <ø> (ø)
nibabel/ecat.py	88.07% <ø> (ø)
nibabel/filename_parser.py	92.04% <ø> (ø)
nibabel/fileslice.py	97.26% <ø> (ø)
nibabel/gifti/gifti.py	95.42% <ø> (ø)
... and 13 more

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[matthew-brett]

Thanks - and yes - an automatic check would be good.