docs: fix invalid jsx in banner block (#11289)

Single quote strings are not supported by our jsx parser

Author: AlessioGr

docs/fields/date.mdx

@@ -239,7 +239,7 @@ This will add a dropdown to the date picker that allows users to select a timezo
 
 You can customise the available list of timezones in the [global admin config](../admin/overview#timezones).
 
-<Banner type='info'>
+<Banner type="info">
   **Good to know:**
   The date itself will be stored in UTC so it's up to you to handle the conversion to the user's timezone when displaying the date in your frontend.