Update file db_diagram.mmd

guides/assets/db_diagram.mmd

+graph TD
+    A[Christmas] -->|Get money| B(Go shopping)
+    B --> C{Let me think}
+    C -->|One| D[Laptop]
+    C -->|Two| E[iPhone]
+    C -->|Three| F[fa:fa-car Car]
\ No newline at end of file