Use string interpolation for test string

Closes #9696

src/NzbDrone.Common/Disk/DiskProviderBase.cs

@@ -130,7 +130,7 @@ public bool FolderWritable(string path)
             try
             {
                 var testPath = Path.Combine(path, "radarr_write_test.txt");
-                var testContent = string.Format("This file was created to verify if '{0}' is writable. It should've been automatically deleted. Feel free to delete it.", path);
+                var testContent = $"This file was created to verify if '{path}' is writable. It should've been automatically deleted. Feel free to delete it.";
                 WriteAllText(testPath, testContent);
                 File.Delete(testPath);
                 return true;