chore: 공연 조회 메서드 이름 변경

- findStageById -> findById

backend/src/main/java/com/festago/admin/presentation/v1/AdminStageV1Controller.java

@@ -29,7 +29,7 @@ public class AdminStageV1Controller {
     private final AdminStageV1QueryService adminStageV1QueryService;
 
     @GetMapping("/{stageId}")
-    public ResponseEntity<AdminStageV1Response> findStageById(
+    public ResponseEntity<AdminStageV1Response> findById(
         @PathVariable  Long stageId
     ) {
         return ResponseEntity.ok()