Typo fixed (missing -r flag)

README.md

@@ -188,7 +188,7 @@ After following the environment setup steps in the previous [section](#environme
 Change directory to `prompt-learning` and install the requirements. Make sure the correct prompt-learning `conda` environment is activated before installing the requirements.  
 ```shell  
 cd prompt-learning
-pip install - requirements.txt
+pip install -r requirements.txt
 ```
 
 ### Train the prompt model