Update 18.create-table.md (#556)

只支持none,cluster

const char* const duplicate_scope_strings[] = {"none",
    //"zone",
    //"region",
    "cluster"};

MySQL [test]> CREATE TABLE test (c1 INT PRIMARY KEY, c2 VARCHAR(50)) REPLICA_NUM = 3,DUPLICATE_SCOPE='region';
ERROR 1210 (HY000): Incorrect arguments to duplicate_scope

MySQL [test]> CREATE TABLE test (c1 INT PRIMARY KEY, c2 VARCHAR(50)) REPLICA_NUM = 3,DUPLICATE_SCOPE='zone';
ERROR 1210 (HY000): Incorrect arguments to duplicate_scope

docs/docs/docs-cn/10.sql-reference/5.sql-statement/18.create-table.md

@@ -74,7 +74,7 @@ table_option:
     | compression
     | AUTO_INCREMENT [=] INT_VALUE
     | comment
-    | DUPLICATE_SCOPE [=] "none|zone|region|cluster"
+    | DUPLICATE_SCOPE [=] "none|cluster"
     | LOCALITY [=] "locality description"
     | ROW_FORMAT [=] REDUNDANT|COMPACT|DYNAMIC|COMPRESSED|DEFAULT
     | PCTFREE [=] num