Update INSTALL.md

For ubuntu focal fossa only instructions given for Eoan worked for me. If we don't add Focal fossa to list people will go to "For other versions" section. which refers to libllvm3.7 which could not be installed on ubuntu focal. Hence this proposed change

INSTALL.md

@@ -385,7 +385,7 @@ sudo apt-get update
 sudo apt-get -y install bison build-essential cmake flex git libedit-dev \
  libllvm6.0 llvm-6.0-dev libclang-6.0-dev python zlib1g-dev libelf-dev
 
-# For Eoan (19.10)
+# For Eoan (19.10) or Focal (20.04.1 LTS)
 sudo apt install -y bison build-essential cmake flex git libedit-dev \
  libllvm7 llvm-7-dev libclang-7-dev python zlib1g-dev libelf-dev