Fix array items order example (rjsf-team#367)

Arrays have `items`, not `properties`.

Author: tkurki

README.md

@@ -415,7 +415,7 @@ Array items are orderable by default, and react-jsonschema-form renders move up/
 ```jsx
 const schema = {
  type: "array",
- properties: {
+ items: {
  type: "string"
  }
 };