fix nebula index ddl

[lybtt]

https://docs.nebula-graph.io/3.8.0/3.ngql-guide/14.native-index-statements/1.create-native-index/#create_single-property_indexes

# To index a variable-length string property, you need to specify the index length.
nebula> CREATE TAG IF NOT EXISTS var_string(p1 string);
nebula> CREATE TAG INDEX IF NOT EXISTS var ON var_string(p1(10));

# To index a fixed-length string property, you do not need to specify the index length.
nebula> CREATE TAG IF NOT EXISTS fix_string(p1 FIXED_STRING(10));
nebula> CREATE TAG INDEX IF NOT EXISTS fix ON fix_string(p1);