Fix environment variable name for Google Scholar API

[rajputshivam2324]

Update API key environment variable reference ,

In credentials , environmental variable is defined like this :
process.env.GOOGLE_SCHOLAR_API_KEY="your-serp-api-key"
to : SERPAPI_API_KEY="your-serp-api-key"

[mdrxy]

Can you provide attribution?

[rajputshivam2324]

In the credentials section, it mentioned using the below code snippets in the .env file.
However, in the .env file, we don’t write process.env.API_KEY_NAME; we only define the key name and its value.

Also, since there’s no official Google Scholar API key, the SERPAPI_API_KEY is actually used for this purpose and the same is referenced later in the docs.
This change helps avoid confusion for beginners who might misunderstand the snippet.