Fix link to MCP Servers config section (#5301)

fix a broken link in
https://github.com/openai/codex/blob/main/codex-rs/config.md to point to
the anchor for configuring MCPs:
https://github.com/openai/codex/blob/main/docs/config.md#connecting-to-mcp-servers

Author: lopopolo-openai

codex-rs/config.md

@@ -3,4 +3,4 @@
 This file has moved. Please see the latest configuration documentation here:
 
 - Full config docs: [docs/config.md](../docs/config.md)
-- MCP servers section: [docs/config.md#mcp_servers](../docs/config.md#mcp_servers) 
+- MCP servers section: [docs/config.md#connecting-to-mcp-servers](../docs/config.md#connecting-to-mcp-servers)