fix the wrong manifest value

The manifest value comes from the previous release being 2.59.0. The
2.59.2 was already made on the main branch:
https://github.com/googleapis/sdk-platform-java/tree/v2.59.2

Author: suztomo

.release-please-manifest.json

@@ -1,3 +1,3 @@
 {
-  ".": "2.59.1"
+  ".": "2.59.3"
 }