Fix basic start tutorial part 2 file references.

[MFraters]

Fixes #771 reported by @anne-glerum. It seems like the other files are fine and just part 2 had the wrong links.

[github-actions]

Benchmark	Main	Feature	Difference (99.9% CI)
Slab interpolation simple none	1.210 ± 0.010 (s=369)	1.210 ± 0.010 (s=377)	-0.1% .. +0.3%
Slab interpolation curved simple none	1.219 ± 0.011 (s=353)	1.220 ± 0.014 (s=388)	-0.2% .. +0.3%
Spherical slab interpolation simple none	1.225 ± 0.009 (s=359)	1.226 ± 0.011 (s=378)	-0.1% .. +0.3%
Slab interpolation simple curved CMS	1.253 ± 0.012 (s=364)	1.254 ± 0.014 (s=357)	-0.2% .. +0.3%
Spherical slab interpolation simple CMS	1.640 ± 0.014 (s=293)	1.644 ± 0.014 (s=258)	-0.0% .. +0.4%
Spherical fault interpolation simple none	1.233 ± 0.011 (s=386)	1.231 ± 0.010 (s=347)	-0.3% .. +0.1%
Cartesian min max surface	2.725 ± 0.024 (s=178)	2.722 ± 0.021 (s=155)	-0.4% .. +0.2%
Spherical min max surface	7.758 ± 0.052 (s=57)	7.757 ± 0.069 (s=62)	-0.5% .. +0.5%