add -r option before "requirements.txt"

docs/en/installation.md

@@ -72,7 +72,7 @@ conda activate xrnerf
 4. check [here](https://pypi.org/project/torchvision/) and install specified vision of torchvision, for example ```pip install torchvision==0.12.0```
 
 #### d. Install Other Needed Python Packages
-* you can use ```pip install requirements.txt``` to install most of the needed pkgs. If this step succeeds, you should jump to ```kilo-cuda``` and ```spconv``` step to install them manually. Or you can skip this step and follow the installation steps below
+* you can use ```pip install -r requirements.txt``` to install most of the needed pkgs. If this step succeeds, you should jump to ```kilo-cuda``` and ```spconv``` step to install them manually. Or you can skip this step and follow the installation steps below
 * ```pip install 'opencv-python>=3' yapf imageio scikit-image lpips trimesh smplx```
 * install ```mmcv-full``` following their [Installation](https://mmcv.readthedocs.io/en/latest/get_started/installation.html)
 * install ```spconv``` using pip install, for example ```pip install spconv-cu111```. notice that only specified cuda-vision are supported, following their [Installation](https://github.com/traveller59/spconv)