doc: update Nix code snippets format

Command: `mdcr --config doc/tests/mdcr-config.toml doc/`

doc/hooks/postgresql-test-hook.section.md

@@ -4,7 +4,11 @@
 This hook starts a PostgreSQL server during the `checkPhase`. Example:
 
 ```nix
-{ stdenv, postgresql, postgresqlTestHook }:
+{
+  stdenv,
+  postgresql,
+  postgresqlTestHook,
+}:
 stdenv.mkDerivation {
 
   # ...
@@ -18,13 +22,13 @@ stdenv.mkDerivation {
 
 If you use a custom `checkPhase`, remember to add the `runHook` calls:
 ```nix
-  checkPhase ''
-    runHook preCheck
+checkPhase ''
+  runHook preCheck
 
-    # ... your tests
+  # ... your tests
 
-    runHook postCheck
-  ''
+  runHook postCheck
+''
 ```
 
 ## Variables {#sec-postgresqlTestHook-variables}