Update 08_passive_margin_variable_depth.md

[ljhwang]

This is the first commit to fix only typos and grammatical errors.

This will be followed by a second commit to modify the paragraph we discussed.

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Benchmark	Main	Feature	Difference (99.9% CI)
Slab interpolation simple none	0.989 ± 0.004 (s=462)	0.989 ± 0.005 (s=450)	-0.1% .. +0.1%
Slab interpolation curved simple none	0.992 ± 0.004 (s=468)	0.991 ± 0.004 (s=442)	-0.1% .. +0.1%
Spherical slab interpolation simple none	1.161 ± 0.005 (s=384)	1.162 ± 0.006 (s=393)	-0.1% .. +0.2%
Slab interpolation simple curved CMS	1.032 ± 0.004 (s=444)	1.033 ± 0.003 (s=430)	-0.0% .. +0.1%
Spherical slab interpolation simple CMS	1.569 ± 0.010 (s=280)	1.570 ± 0.013 (s=296)	-0.1% .. +0.3%
Spherical fault interpolation simple none	1.163 ± 0.013 (s=393)	1.163 ± 0.007 (s=383)	-0.3% .. +0.1%
Cartesian min max surface	2.371 ± 0.027 (s=185)	2.371 ± 0.022 (s=197)	-0.3% .. +0.4%
Spherical min max surface	7.041 ± 0.082 (s=57)	7.032 ± 0.037 (s=73)	-0.7% .. +0.4%