[MINOR] docs: correct json body in create_table (#2923)

### What changes were proposed in this pull request?

correct json body in create_table

### Why are the changes needed?

HTTP request body is illegal, miss a dot in body

use this command may get an exception message

### Does this PR introduce _any_ user-facing change?

N/A

### How was this patch tested?

Mannal

docs/manage-relational-metadata-using-gravitino.md

@@ -477,7 +477,7 @@ curl -X POST -H "Accept: application/vnd.gravitino.v1+json" \
     },
     {
       "name": "name",
-      "type": "varchar(500)"
+      "type": "varchar(500)",
       "comment": "name column comment",
       "nullable": true,
       "autoIncrement": false,