[Doc] Fix an incorrect URL

This PR fixes the following incorrect URL:
https://spec.MCP.io/specification/2025-03-26/basic/utilities/ping/#behavior-requirements

The specification version used in the official links follows "2025-06-18,"
which is the version consistently used throughout the README.md.

test/mcp/server_test.rb

@@ -72,7 +72,7 @@ module MCP
       )
     end
 
-    # https://spec.MCP.io/specification/2025-03-26/basic/utilities/ping/#behavior-requirements
+    # https://modelcontextprotocol.io/specification/2025-06-18/basic/utilities/ping#behavior-requirements
     test "#handle ping request returns empty response" do
       request = {
         jsonrpc: "2.0",