Update geometry.py docstring from compositor to modifier

[mherbertson]

This changes the docstring example from compositor to modifier.

[codecov]

Codecov Report

Merging #1988 (29b926c) into main (cccbe06) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1988   +/-   ##
=======================================
  Coverage   93.54%   93.54%           
=======================================
  Files         279      279           
  Lines       41322    41322           
=======================================
  Hits        38654    38654           
  Misses       2668     2668           

Flag	Coverage Δ
behaviourtests	4.78% <ø> (ø)
unittests	94.07% <ø> (ø)

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Impacted Files	Coverage Δ
satpy/modifiers/geometry.py	87.30% <ø> (ø)

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[coveralls]

Coverage remained the same at 94.005% when pulling 29b926c on mherbertson:fix-geometry-docstring into cccbe06 on pytroll:main.

[pnuu]

LGTM