Quote @parent_yaml directive for syntax compliance.

config/vufind/searchspecs.yaml

@@ -154,7 +154,7 @@
 # Note that you may create a "@parent_yaml" entry at the very top of the file to
 # inherit sections from another file. For example:
 #
-# @parent_yaml: "/path/to/my/file.yaml"
+# "@parent_yaml": "/path/to/my/file.yaml"
 #
 # Only sections not found in this file will be loaded in from the parent file.
 # In some complex scenarios, this can be a useful way of sharing settings

module/VuFind/tests/unit-tests/src/VuFindTest/Config/SearchSpecsReaderTest.php

@@ -65,9 +65,9 @@ class SearchSpecsReaderTest extends \VuFindTest\Unit\TestCase
         $parentPath = Locator::getLocalConfigPath('top.yaml', null, true);
         $parent = "top: foo";
         $childPath = Locator::getLocalConfigPath('middle.yaml', null, true);
-        $child = "@parent_yaml: $parentPath\nmiddle: bar";
+        $child = "\"@parent_yaml\": $parentPath\nmiddle: bar";
         $grandchildPath = Locator::getLocalConfigPath('bottom.yaml', null, true);
-        $grandchild = "@parent_yaml: $childPath\nbottom: baz";
+        $grandchild = "\"@parent_yaml\": $childPath\nbottom: baz";
 
         // Fail if we are unable to write files:
         if (null === $parentPath || null === $childPath || null === $grandchildPath