Fix asciicast image paths for dark mode

[yjunechoe]

markdown:

jlmerclusterperm/README.md

Lines 66 to 69 in 13f7643

	<picture>
	<source media="(prefers-color-scheme: dark)" srcset="man/figures/README-/setup-io-dark.svg">
	<img src="man/figures/README-/setup-io.svg" style="display: block; margin: auto;" />
	</picture>

pkgdown html:

<picture>
  <source media="(prefers-color-scheme: dark)" srcset="man/figures/README-/setup-io-dark.svg">
  <img src="reference/figures/README-/setup-io.svg" style="display: block; margin: auto;"></source>
</picture>

jlmerclusterperm/docs/index.html

Line 118 in 13f7643

<p><picture><source media="(prefers-color-scheme: dark)" srcset="man/figures/README-/setup-io-dark.svg"><img src="reference/figures/README-/setup-io.svg" style="display: block; margin: auto;"></source></picture></p>

[yjunechoe]

Also an option to render asciicast to html directly, stead of svg - https://fosstodon.org/@gaborcsardi/111962906731180313

[yjunechoe]

Resolved upstream in pkgdown