Update API key environment variable name in Readme.md

[Med-Yassine-B]

HI ✌️ i noticed a typo in the README.md file while trying to run the project
In the build instructions the API key name in the .env is written like this

api_key=the_actual_api_key

while the python code expects
EXCHANGE_RATE_API_KEY=the_actual_api_key

What changed?

• Updated the README.md

Related issue

// no related issue

Tests

• I ran pytest
• Existing tests pass
• I added or updated tests where necessary
• Not needed because this only affects documentation or repository setup

Checklist

• The change is focused
• No secrets or API keys are included
• Documentation was updated if needed
• CI is expected to pass

Notes

[BytecodeBrewer]

@Med-Yassine-B You're right. I forgot to check what the prod code expects to receive
Edit: Please amend your commit because it doesn't follow the commit convention.

Example:

git commit --amend -m "fix(#42): update the api key variable name in readme"
git push --force-with-lease

[Med-Yassine-B]

@BytecodeBrewer It looks like i missed the commit workflow on CONTRIBUTING.md
i edited my commit as required

[BytecodeBrewer]

@Med-Yassine-B Great, now I merged it.