Move postgresql installation command into code block

Author: andocz

doc/usage.md

@@ -15,9 +15,15 @@ creating and using the database file.
 
 ### Using *PostgreSQL* from package manager
 
-PostgreSQL can be installed from the package manager, using
-`sudo apt install postgresql-<version>` (e.g. `postgresql-9.5`). This will
-set up an automatically starting local server on the default port `5432`.
+PostgreSQL can be installed from the package manager:
+
+```bash
+sudo apt install postgresql-<version>
+# (e.g. postgresql-12)
+```
+
+This will set up an automatically starting local server on the default port
+`5432`.
 
 This server, by default, is only accessible for an automatically created system
 user named `postgres`. However, CodeCompass's database layer only supports