Update README.md (#2581)

* Update README.md

type in pip command to install requirements

* Update model_store.py

fix not working url. the models in the previous url were just stub.

python/README.md

@@ -70,7 +70,7 @@ mat = ncnn.Mat(mat_np)
 # Model Zoo
 install requirements
 ```bash
-pip install -U requirements.txt
+pip install -r requirements.txt
 ```
 then you can import ncnn.model_zoo and get model list as follow:
 ```bash
@@ -83,4 +83,4 @@ all model in model zoo has example in ncnn/python/examples folder
 
 # Custom Layer
 
-custom layer demo is in ncnn/python/ncnn/model_zoo/yolov5.py:23
+custom layer demo is in ncnn/python/ncnn/model_zoo/yolov5.py:23

python/ncnn/model_zoo/model_store.py

@@ -64,7 +64,7 @@ _model_sha1 = {
     ]
 }
 
-github_repo_url = "https://github.com/caishanli/pyncnn-assets/raw/master/models/"
+github_repo_url = "https://github.com/nihui/ncnn-assets/raw/master/models/"
 _url_format = "{repo_url}{file_name}"