Update cast.md

Author: Chasen-Zhang

docs/en/sql-reference/20-sql-functions/02-conversion-functions/cast.md

@@ -28,7 +28,7 @@ SELECT CAST(1 AS VARCHAR), 1::VARCHAR;
 
 
 Cast String to
-Variant and Cast Variant to Map<String, Variant>
+Variant and Cast Variant to `Map<String, Variant>`
 ```sql
 select '{"k1":"v1","k2":"v2"}'::Variant a, a::Map(String, String) b, b::Variant = a;
 ┌──────────────────────┬──────────────────────┬────────────────┐