Minor doc typo fixes on non linear measurement models

[sdhiscocks]

Raised in #605

[codecov]

Codecov Report

Merging #606 (0072ac7) into main (071abc8) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #606   +/-   ##
=======================================
  Coverage   94.37%   94.37%           
=======================================
  Files         154      154           
  Lines        7443     7443           
  Branches     1414     1414           
=======================================
  Hits         7024     7024           
  Misses        315      315           
  Partials      104      104           

Flag	Coverage Δ
integration	68.01% <ø> (+0.02%)	⬆️
unittests	91.75% <ø> (ø)

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Impacted Files	Coverage Δ
stonesoup/models/measurement/nonlinear.py	98.94% <ø> (ø)

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