INSTALL.md: add missing ```sh code fences

docs/INSTALL.md

@@ -90,7 +90,9 @@ package on your machine.
 If you are on the Edge channel of Alpine Linux or postmarketOS, Mirage can be
 installed right from the testing repositry:
 
+```sh
 apk add mirage
+```
 
 If you are unsure about what Edge is and want to read more about it, you can do
 so on the [Alpine Wiki](https://wiki.alpinelinux.org/wiki/Edge).
@@ -105,15 +107,19 @@ available.
 Installing the release version with a AUR helper, e.g. 
 [yay](https://github.com/Jguer/yay):
 
+```sh
 yay -S matrix-mirage
+```
 
 #### Debian
 
 Requires [Debian Testing](https://wiki.debian.org/DebianTesting). 
 To install the package:
 
+```sh
 apt update
 apt install matrix-mirage
+```
 
 #### Gentoo
 
@@ -131,18 +137,24 @@ Installing Mirage:
 
 Requires the unstable channel, to add it:
 
+```sh
 nix-channel --add https://nixos.org/channels/nixpkgs-unstable
 nix-channel --update
+```
 
 To install the package:
 
+```sh
 nix-env -iA nixpkgs.mirage-im
+```
 
 #### OpenMandriva Lx
 
 Requires [Unstable or Rolling][1]. To install the package:
 
+```sh
 sudo dnf install matrix-mirage
+```
 
 [1]: https://openmandriva.net/wiki/en/index.php?title=OpenMandriva_Release_Plan_and_Repositories#Release_Plan