Fix indentation + comment wrt paths_ignore

.github/workflows/shellcheck.yml

@@ -1,6 +1,10 @@
+# Runs shellcheck on code -- in error mode only.
+# "paths-ignore" should be in line with unit_test.*yml files
 ---
 name: Shellcheck
-on: [pull_request]
+
+on:
+  pull_request:
     paths-ignore:
       - 'utils/**'
       - 'doc/**'
@@ -14,6 +18,7 @@ on: [pull_request]
       - 'LICENSE'
       - 'Dockerfile'
       - 'Dockerfile.alpine'
+
 permissions:
   contents: read