doc: fix better doc for api develop, droping dead hint (#10906)

Signed-off-by: yihong0618 <zouzou0208@gmail.com>

api/README.md

    ```
 
 2. Copy `.env.example` to `.env`
+
+   ```cli
+   cp .env.example .env 
+   ```
 3. Generate a `SECRET_KEY` in the `.env` file.
 
+   bash for Linux
    ```bash for Linux
    sed -i "/^SECRET_KEY=/c\SECRET_KEY=$(openssl rand -base64 42)" .env
    ```
-
+   bash for Mac
    ```bash for Mac
    secret_key=$(openssl rand -base64 42)
    sed -i '' "/^SECRET_KEY=/c\\
    poetry install
    ```
 
-   In case of contributors missing to update dependencies for `pyproject.toml`, you can perform the following shell instead.
-
-   ```bash
-   poetry shell                                               # activate current environment
-   poetry add $(cat requirements.txt)           # install dependencies of production and update pyproject.toml
-   poetry add $(cat requirements-dev.txt) --group dev    # install dependencies of development and update pyproject.toml
-   ```
-
 6. Run migrate
 
    Before the first launch, migrate the database to the latest version.