[docs](hive)(function) fix Hive type error and optimize alias function example (#7694)

1. fix Hive type error 
2. optimize alias function example

docs/en/sql-reference/sql-statements/Data Definition/create-function.md

@@ -135,8 +135,8 @@ If the `function_name` contains the database name, the custom function will be c
         AS CONCAT(LEFT(id, 3), '****', RIGHT(id, 4));
 
     -- create a custom cast alias function
-    CREATE ALIAS FUNCTION decimal(ALL, INT, INT) WITH PARAMETER(col, precision, scale) 
-        AS CAST(col AS decimal(precision, scale));
+    CREATE ALIAS FUNCTION string(ALL, INT) WITH PARAMETER(col, length) 
+        AS CAST(col AS varchar(length));
     ```
 
 ## keyword