Update section Using node-postgres in Express in the Using PostgreSQL course

[Mohamedsayhii]

Because

Forgetting to change the <role_name> and <role_password> in db/pool.js file can lead to confusion not knowing what went wrong when running the app.

This PR

• Added a phrase to learners reminding them to change the variables <role_name> and <role_password> in the db/pool.js file after creating it.

Issue

Closes #XXXXX

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