[Doc] Fix incorrect schema name

This PR fixes an incorrect schema name.

The test code already uses `items:`.
https://github.com/modelcontextprotocol/ruby-sdk/blob/v0.4.0/test/mcp/tool/output_schema_test.rb#L135-L141

Fixes #170.

Author: koic

README.md

@@ -551,7 +551,7 @@ Output schema may also describe an array of objects:
 class WeatherTool < MCP::Tool
   output_schema(
     type: "array",
-    item: {
+    items: {
       properties: {
         temperature: { type: "number" },
         condition: { type: "string" },