Update figure caption 04_adding_first_tectonic_feature.md

[anne-glerum]

Clarify figure caption

[github-actions]

Benchmark	Main	Feature	Difference (99.9% CI)
Slab interpolation simple none	1.214 ± 0.012 (s=376)	1.212 ± 0.010 (s=368)	-0.4% .. +0.1%
Slab interpolation curved simple none	1.216 ± 0.010 (s=368)	1.217 ± 0.013 (s=374)	-0.2% .. +0.3%
Spherical slab interpolation simple none	1.229 ± 0.012 (s=324)	1.229 ± 0.011 (s=411)	-0.3% .. +0.2%
Slab interpolation simple curved CMS	1.254 ± 0.017 (s=359)	1.254 ± 0.010 (s=361)	-0.3% .. +0.3%
Spherical slab interpolation simple CMS	1.637 ± 0.014 (s=273)	1.636 ± 0.014 (s=279)	-0.3% .. +0.1%
Spherical fault interpolation simple none	1.230 ± 0.012 (s=364)	1.228 ± 0.012 (s=371)	-0.3% .. +0.1%
Cartesian min max surface	2.707 ± 0.025 (s=171)	2.705 ± 0.021 (s=164)	-0.4% .. +0.2%
Spherical min max surface	7.760 ± 0.107 (s=66)	7.757 ± 0.066 (s=53)	-0.7% .. +0.7%

[MFraters]

Thanks! That makes it more clear.