[docs] update Gentoo installation instructions. (#4227)

layman has been removed from the main Gentoo Portage and has been
replaced with eselect repository.

Signed-off-by: Tomas Fabrizio Orsi <torsi@fi.uba.ar>

Author: lima-limon-inc

download.dd

@@ -172,7 +172,7 @@ $(BR)$(BR)
 
 $(DOWNLOAD_OTHER $(ARCHLINUX), $(LINK2 https://wiki.archlinux.org/index.php/D_(programming_language), Arch Linux), $(CONSOLE pacman -S dlang))
 
-$(DOWNLOAD_OTHER $(GENTOO), $(LINK2 https://wiki.gentoo.org/wiki/Dlang, Gentoo), $(CONSOLE layman -f -a dlang))
+$(DOWNLOAD_OTHER $(GENTOO), $(LINK2 https://wiki.gentoo.org/wiki/Dlang, Gentoo), $(CONSOLE eselect repository enable dlang))
 
 $(DOWNLOAD_OTHER $(HOMEBREW), $(LINK2 https://formulae.brew.sh/formula/dmd, Homebrew), $(CONSOLE brew install dmd))