refactor: email API 경로명 수정

backend/src/main/java/com/edufocus/edufocus/mail/controller/MailController.java

@@ -18,7 +18,7 @@ public class MailController {
 
     private final UserService userService;
 
-    @PostMapping("/sendCode")
+    @PostMapping("/sendcode")
     public ResponseEntity<?> sendMail(@RequestParam String email) {
         if (!userService.isEmailExist(email)) {
             return new ResponseEntity<>(HttpStatus.NOT_FOUND);