Update 08_passive_margin_variable_depth.md

[ljhwang]

Here is an attempt to fix the paragraph under discussion.

Please feel free to edit.

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Benchmark	Main	Feature	Difference (99.9% CI)
Slab interpolation simple none	1.014 ± 0.005 (s=469)	1.021 ± 0.003 (s=418)	+0.6% .. +0.8%
Slab interpolation curved simple none	1.017 ± 0.004 (s=455)	1.023 ± 0.006 (s=430)	+0.4% .. +0.7%
Spherical slab interpolation simple none	1.188 ± 0.007 (s=378)	1.188 ± 0.007 (s=382)	-0.1% .. +0.1%
Slab interpolation simple curved CMS	1.058 ± 0.005 (s=434)	1.060 ± 0.005 (s=418)	+0.1% .. +0.3%
Spherical slab interpolation simple CMS	1.596 ± 0.010 (s=285)	1.598 ± 0.011 (s=281)	-0.1% .. +0.3%
Spherical fault interpolation simple none	1.193 ± 0.006 (s=348)	1.193 ± 0.012 (s=409)	-0.2% .. +0.2%
Cartesian min max surface	2.326 ± 0.028 (s=186)	2.330 ± 0.026 (s=203)	-0.2% .. +0.6%
Spherical min max surface	7.231 ± 0.030 (s=62)	7.236 ± 0.032 (s=65)	-0.2% .. +0.3%