Update CONTRIBUTING.md (#1520)

CONTRIBUTING.md

@@ -146,7 +146,7 @@ instead of forking it.
 ### Development environment
 
 This repository contains several projects with different technologies. Depending on the project that you selected (eg. gateway), from the project directory you will run:
-- `pip install -r requirements.txt requirements-dev.txt` for python projects (strongly consider using a [virtual environment](https://docs.python.org/3/library/venv.html)!).
+- `pip install -r requirements.txt -r requirements-dev.txt` for python projects (strongly consider using a [virtual environment](https://docs.python.org/3/library/venv.html)!).
 - `helm dependency build` for helm (Before running this command, make sure to check for helm configuration instructions specific to your selected project charts).
 -  `terraform init` for terraform.