Update WSL install procedure for 22-Titanium

INSTALL.md

@@ -129,7 +129,7 @@ following tools:
 #### Prerequisites: WSL + a Linux distribution
 
 For enabling WSL on Windows, you may follow these instructions
-(we tested with Ubuntu 18.04 LTS;
+(we tested with Ubuntu 20.04 LTS;
 other distributions/versions should also work,
 but the instructions below may require some modifications).
 
@@ -158,7 +158,7 @@ Move to your user directory, download the distribution and install it:
 
 ```
 cd C:\Users\<Your User Directory>
-Invoke-WebRequest -Uri https://aka.ms/wsl-ubuntu-1804 -OutFile Ubuntu.appx -UseBasicParsing
+Invoke-WebRequest -Uri https://aka.ms/wslubuntu2004 -OutFile Ubuntu.appx -UseBasicParsing
 Add-AppxPackage .\Ubuntu.appx
 ```
 
@@ -171,10 +171,9 @@ For installing opam, some packages are required. The following commands can be
 run to update the system and install those packages:
 
 ```
-sudo add-apt-repository -y ppa:avsm/ppa # unnecessary for Ubuntu 20.04
 sudo apt update
 sudo apt upgrade
-sudo apt install make m4 gcc opam
+sudo apt install make m4 gcc opam yaru-theme-gtk yaru-theme-icon
 ```
 
 Then opam can be set up using these commands: