Update architecture diagram host label (modelcontextprotocol#153)

Co-authored-by: David Soria Parra <[email protected]>

Author: KajiyamaVK

docs/concepts/architecture.mdx

@@ -15,7 +15,7 @@ MCP follows a client-server architecture where:
 
 ```mermaid
 flowchart LR
-    subgraph " Host "
+    subgraph "Host"
         client1[MCP Client]
         client2[MCP Client]
     end