Better styling in upgrade guide

UPGRADE_GUIDE.md

@@ -6,53 +6,53 @@
 
 1. Attach the new logger through telemetry events in your project's `application.ex` file:
 
-```elixir
-# lib/pixie/application.ex
-def start(_type, _args) do
-  children = [
-    # ...
-  ]
-
-  #
-  # Add the line below:
-  #
-  :ok = Logster.attach_phoenix_logger()
-
-  opts = [strategy: :one_for_one, name: MyApp.Supervisor]
-  Supervisor.start_link(children, opts)
-end
-```
+   ```elixir
+   # lib/pixie/application.ex
+   def start(_type, _args) do
+     children = [
+       # ...
+     ]
+
+     #
+     # Add the line below:
+     #
+     :ok = Logster.attach_phoenix_logger()
+
+     opts = [strategy: :one_for_one, name: MyApp.Supervisor]
+     Supervisor.start_link(children, opts)
+   end
+   ```
 
 2. Locate the line installing `Logster.Plugs.Logger` in your `endpoint.ex` file.
 
-```elixir
-plug Logster.Plugs.Logger,
-  # Configuration options such as those below might not be present
-  formatter: Logster.Plugs.JSONFormatter,
-  allowed_headers: ["content-type"],
-  excludes: [:params]
-```
+   ```elixir
+   plug Logster.Plugs.Logger,
+     # Configuration options such as those below might not be present
+     formatter: Logster.Plugs.JSONFormatter,
+     allowed_headers: ["content-type"],
+     excludes: [:params]
+   ```
 
 3. If configuration options were passed to `Logster.Plugs.Logger`, move them to `config.exs`:
 
-```elixir
-config :logster,
-  formatter: :json,
-  headers: ["content-type"],
-  excludes: [:params]
-```
+   ```elixir
+   config :logster,
+     formatter: :json,
+     headers: ["content-type"],
+     excludes: [:params]
+   ```
 
-NOTE: `allowed_headers` option has been renamed to `headers`
+   > NOTE: `allowed_headers` option has been renamed to `headers`
 
 4. Remove the line installing `Logster.Plugs.Logger` from your `endpoint.ex` file.
 
 5. Locate any calls to `Logster.Plugs.ChangeLogLevel` and rename it to `Logster.ChangeLogLevel`
 
 6. Add the following to `config.exs` to disable the default Phoenix logger:
 
-```elixir
-config :phoenix, :logger, false
-```
+   ```elixir
+   config :phoenix, :logger, false
+   ```
 
 ### Plug users: