docs: Add missing curly bracket (#1297)

Just copied some example code into my own code and noticed a missing
curly bracket there.

docs/pages/docs/usage/messages.mdx

@@ -309,7 +309,7 @@ Values are required to be alphanumeric and can contain underscores. All other ch
 Therefore, e.g. when you're mapping a locale to a human readable string, you should map the dash to an underscore first:
 
 ```tsx filename="en.json"
-"label": "{locale, select, en_GB {British English} en_US {American English} other {Unknown}"
+"label": "{locale, select, en_GB {British English} en_US {American English} other {Unknown}}"
 ```
 
 ```tsx