Escape vertical bar in CustomCheckbox Props table (#871)

Author: zgotsch

website/src/pages/checkbox.mdx

@@ -431,7 +431,7 @@ Please see the [styling guide](/styling).
 | [`disabled`](#customcheckbox-disabled)             | `boolean`                    | false    |
 | [`name`](#customcheckbox-name)                     | `string`                     | false    |
 | [`onChange`](#customcheckbox-onchange)             | `func`                       | false    |
-| [`value`](#customcheckbox-value)                   | `string | number`            | false    |
+| [`value`](#customcheckbox-value)                   | `string \| number`           | false    |
 
 ##### CustomCheckbox `span` props