docs: update WeChat group 18 QR code (#370)

README.md

@@ -264,7 +264,7 @@ You're also welcome to join our **GenericAgent Community Group** for discussion,
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-      <td align="center"><strong>WeChat Group 17</strong><br><img src="assets/images/wechat_group17.jpg" alt="WeChat Group 17 QR Code" width="250"/></td>
+      <td align="center"><strong>WeChat Group 18</strong><br><img src="assets/images/wechat_group18.jpg" alt="WeChat Group 18 QR Code" width="250"/></td>
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   </table>
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@@ -606,7 +606,7 @@ GenericAgent 通过**分层记忆 × 最小工具集 × 自主执行循环**完
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-      <td align="center"><strong>微信群 17</strong><br><img src="assets/images/wechat_group17.jpg" alt="微信群 17 二维码" width="250"/></td>
+      <td align="center"><strong>微信群 18</strong><br><img src="assets/images/wechat_group18.jpg" alt="微信群 18 二维码" width="250"/></td>
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