rename backend -> app

README.md

@@ -41,14 +41,14 @@ Some basic steps to setup the project and its dependencies.
     - Debian: `apt install -y inotify-tools`
 - Setup elixir
     - Install hex: `mix local.hex`
-    - `cd backend && mix escript.install hex protobuf`
+    - `cd app && mix escript.install hex protobuf`
     - `asdf reshim`
 
 ## Preparing for development
 
-### Backend
+### Elixir app
 
-- `cd backend`
+- `cd app`
 - Start the database: `docker-compose up -d`
 - `mix setup`
 - `mix phx.server`