Use zoned offset for the :datetime literal example

Co-authored-by: Richard Gibson <richard.gibson@gmail.com>

Author: stasm

spec/syntax.md

@@ -378,7 +378,7 @@ Examples:
 ```
 
 ```
-|Thu Jan 01 1970 14:37:00 GMT+0100 (CET)| :datetime weekday=long
+|1970-01-01T14:37:00+01:00[Europe/Berlin]| :datetime weekday=long
 ```
 
 ```