Update 18_2D_models.md

[ljhwang]

Fixes typos.

MAJOR edit to description of first figure. @MFraters, please check.

Also, in the first note, is the following correct:
... the origin of the 2D cross section is located at [0,200e3] ...

Should it be 450e3 ?

[github-actions]

Benchmark	Main	Feature	Difference (99.9% CI)
Slab interpolation simple none	0.992 ± 0.006 (s=436)	0.992 ± 0.007 (s=474)	-0.2% .. +0.1%
Slab interpolation curved simple none	0.983 ± 0.010 (s=453)	0.983 ± 0.010 (s=465)	-0.2% .. +0.2%
Spherical slab interpolation simple none	1.158 ± 0.007 (s=392)	1.158 ± 0.007 (s=387)	-0.1% .. +0.1%
Slab interpolation simple curved CMS	1.035 ± 0.005 (s=452)	1.035 ± 0.004 (s=420)	-0.1% .. +0.1%
Spherical slab interpolation simple CMS	1.572 ± 0.009 (s=285)	1.573 ± 0.010 (s=290)	-0.1% .. +0.2%
Spherical fault interpolation simple none	1.167 ± 0.008 (s=385)	1.167 ± 0.012 (s=389)	-0.2% .. +0.2%
Cartesian min max surface	2.373 ± 0.022 (s=186)	2.371 ± 0.014 (s=196)	-0.4% .. +0.2%
Spherical min max surface	7.017 ± 0.032 (s=70)	7.024 ± 0.087 (s=61)	-0.5% .. +0.7%