Fix documentation reference to use lowercase file extension for README.md

Author: roryp

04-PracticalSamples/mcp/calculator/src/main/java/com/microsoft/mcp/sample/server/controller/HealthController.java

@@ -96,7 +96,7 @@ public ResponseEntity<Map<String, Object>> serviceInfo() {
         response.put("availableTools", tools);
 
         // Additional helpful information for clients
-        response.put("documentation", "See README.MD for usage examples");
+        response.put("documentation", "See README.md for usage examples");
         response.put("clientExamples", "Check src/test/java for client implementation examples");
 
         return ResponseEntity.ok(response);