docs: fix default value of cache_seconds query option in README (anur…

…aghazra#3644)

readme.md

@@ -297,7 +297,7 @@ You can customize the appearance of all your cards however you wish with URL par
 | `bg_color` | Card's background color. | string (hex color or a gradient in the form of *angle,start,end*) | `fffefe` |
 | `hide_border` | Hides the card's border. | boolean | `false` |
 | `theme` | Name of the theme, choose from [all available themes](themes/README.md). | enum | `default` |
-| `cache_seconds` | Sets the cache header manually (min: 21600, max: 86400). | integer | `1800` |
+| `cache_seconds` | Sets the cache header manually (min: 21600, max: 86400). | integer | `21600` |
 | `locale` | Sets the language in the card, you can check full list of available locales [here](#available-locales). | enum | `en` |
 | `border_radius` | Corner rounding on the card. | number | `4.5` |