refactor: 테마 DTO의 thumbnail에는 Schema 추가

src/main/kotlin/roomescape/theme/web/ThemeDTO.kt

@@ -1,5 +1,6 @@
 package roomescape.theme.web
 
+import io.swagger.v3.oas.annotations.media.Schema
 import jakarta.validation.constraints.NotBlank
 import jakarta.validation.constraints.Size
 import org.hibernate.validator.constraints.URL
@@ -16,6 +17,7 @@ data class ThemeCreateRequest(
 
         @URL
         @NotBlank
+        @Schema(description = "썸네일 이미지 주소(URL).")
         val thumbnail: String
 )
 
@@ -29,6 +31,7 @@ data class ThemeRetrieveResponse(
         val id: Long,
         val name: String,
         val description: String,
+        @Schema(description = "썸네일 이미지 주소(URL).")
         val thumbnail: String
 )