minor improvements to Debian/Ubuntu based install guide (#1731)

$ mix hex.info

Hex:    2.0.6
Elixir: 1.15.7
OTP:    26.1.2

Built with: Elixir 1.14.2 and OTP 23.3

Author: dch

install.markdown

@@ -60,7 +60,7 @@ Keep in mind that each Elixir version supports specific Erlang/OTP versions. [Se
       ```bash
       $ sudo add-apt-repository ppa:rabbitmq/rabbitmq-erlang
       $ sudo apt update
-      $ sudo apt install elixir
+      $ sudo apt install elixir erlang-dev erlang-xmerl
       ```
 
   - **Fedora 21 (and older)**