_content/blog/llmpowered: set background for the diagram

Adding the white background improves displaying the RAG server diagram
on the dark theme.

Fixes golang/go#69517

Change-Id: Icbe6248280814a2890667bd6a226722226ad9a74
Reviewed-on: https://go-review.googlesource.com/c/website/+/613435
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Eli Bendersky <eliben@google.com>
TryBot-Bypass: Eli Bendersky <eliben@google.com>
diff --git a/_content/blog/llmpowered/rag-server-diagram.png b/_content/blog/llmpowered/rag-server-diagram.png
index b585b85..15f575f 100644
--- a/_content/blog/llmpowered/rag-server-diagram.png
+++ b/_content/blog/llmpowered/rag-server-diagram.png
Binary files differ