docs: replace archived servers in example config with active ones

[alvabillwu]

The "Additional examples" Claude Desktop configuration in the README includes github and postgres server entries, but both of these servers are listed as archived in the same README's Archived section.

This replaces them with time and memory, which are active reference implementations, keeping the example relevant and not pointing users at unmaintained servers.

Before

{
  "mcpServers": {
    "filesystem": { ... },
    "git": { ... },
    "github": {
      "command": "npx",
      "args": ["-y", "@modelcontextprotocol/server-github"],
      "env": {
        "GITHUB_PERSONAL_ACCESS_TOKEN": "<YOUR_TOKEN>"
      }
    },
    "postgres": {
      "command": "npx",
      "args": ["-y", "@modelcontextprotocol/server-postgres", "postgresql://localhost/mydb"]
    }
  }
}

After

{
  "mcpServers": {
    "filesystem": { ... },
    "git": { ... },
    "time": {
      "command": "npx",
      "args": ["-y", "@modelcontextprotocol/server-time"]
    },
    "memory": {
      "command": "npx",
      "args": ["-y", "@modelcontextprotocol/server-memory"]
    }
  }
}