add $ signs in the README backend instructions

README.md

@@ -5,9 +5,9 @@
 You can start a virtual environment with the following instructions.
 
 ```sh
-cd backend
-python3.9 -m venv env
-source env/bin/activate
+$ cd backend
+$ python3.9 -m venv env
+$ source env/bin/activate
 ```
 
 Then you need to install the dependencies by executing `pip install -r requirements.txt`