docs: align buzz-acp box in README architecture diagram

[melvincarvalho]

The buzz-acp box in the README architecture diagram has its right border 2 characters short of the box outline, so the box renders misaligned:

┌──────────────┐
│  buzz-acp  │
│  (ACP ↔ MCP) │
└──────┬───────┘

This pads the label line to the same interior width as the rest of the box:

┌──────────────┐
│   buzz-acp   │
│  (ACP ↔ MCP) │
└──────┬───────┘