Resolve #3408488 "Make unknownpathexcluder accept"

Closes #3408488

package_manager/src/Plugin/Validation/Constraint/RelativePathConstraint.php

@@ -17,6 +17,6 @@ final class RelativePathConstraint extends Constraint {
   /**
    * {@inheritdoc}
    */
-  public string $message = '{{ value }} must be a relative path.';
+  public string $message = 'This value must be a relative path.';
 
 }