fix typo in documentation file: 03_optional_coordinate_system.md

[SabatinoSantangelo]

No description provided.

[danieldouglas92]

Looks great!

[github-actions]

Benchmark	Main	Feature	Difference (99.9% CI)
Slab interpolation simple none	1.124 ± 0.011 (s=407)	1.125 ± 0.011 (s=396)	-0.2% .. +0.3%
Slab interpolation curved simple none	1.127 ± 0.012 (s=400)	1.125 ± 0.009 (s=402)	-0.3% .. +0.1%
Spherical slab interpolation simple none	1.097 ± 0.008 (s=437)	1.099 ± 0.009 (s=385)	-0.1% .. +0.3%
Slab interpolation simple curved CMS	1.165 ± 0.013 (s=389)	1.166 ± 0.013 (s=386)	-0.2% .. +0.3%
Spherical slab interpolation simple CMS	1.443 ± 0.013 (s=302)	1.443 ± 0.013 (s=324)	-0.2% .. +0.3%
Spherical fault interpolation simple none	1.106 ± 0.009 (s=412)	1.106 ± 0.009 (s=404)	-0.2% .. +0.2%
Cartesian min max surface	2.542 ± 0.017 (s=178)	2.546 ± 0.020 (s=178)	-0.1% .. +0.4%
Spherical min max surface	7.182 ± 0.080 (s=63)	7.178 ± 0.072 (s=65)	-0.7% .. +0.6%