add unetformer code

Author: liaoxin2

docs/zh/examples/unetformer.md

@@ -220,4 +220,4 @@ examples/unetformer/vaihingen_test.py:61:121
 ## 6. 参考文献
 
 - [UNetFormer: A UNet-like Transformer for Efficient Semantic Segmentation of Remote Sensing Urban Scene Imagery](https://arxiv.org/abs/2109.08937)
-- [https://github.com/WangLibo1995/GeoSeg](https://github.com/WangLibo1995/GeoSeg)
+- [https://github.com/WangLibo1995/GeoSeg](https://github.com/WangLibo1995/GeoSeg)