[docs] deepseek-tui: China endpoint `api.deepseeki.com` is a non-existent domain (typo)

## Problem / 问题

In `docs/deepseek-tui.md` (line 71) and `docs/deepseek-tui.zh-CN.md` (line 71), the `DEEPSEEK_BASE_URL` row tells users to use
`https://api.deepseeki.com` for the "China endpoint":

> | `DEEPSEEK_BASE_URL` | API base URL — defaults to `https://api.deepseek.com`; use `https://api.deepseeki.com` for the China
endpoint |

This is wrong on two counts:

1. **`api.deepseeki.com` is a typo** (extra `i`) and the domain does not resolve:
```
$ nslookup api.deepseeki.com
*** Non-existent domain
$ nslookup api.deepseek.com
Addresses: 36.159.70.231, 120.226.37.16 # resolves fine
```
2. **DeepSeek has no separate "China endpoint."** The official API base URL is only `https://api.deepseek.com`.

## Impact / 影响

Any user who sets `DEEPSEEK_BASE_URL=https://api.deepseeki.com` as instructed will get DNS/connection failures on **every**
API request — the tool cannot reach DeepSeek at all.

## Suggested fix / 修复建议

Remove the misleading "China endpoint" clause entirely (or, if a China-region URL genuinely exists, replace it with the
correct one). The row should simply read:

> | `DEEPSEEK_BASE_URL` | API base URL — defaults to `https://api.deepseek.com` |

Both the English and zh-CN files need the same fix.

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