GCP credential file wrong documentation

Hi all. When validating the gcp using credential file, Documentation asks to provide in following format
```
{
"project": "",
"client_email": "",
"private_key": ""
}
```
But, according to `index.js` file, if you are passing a credential file, you are expected to provide
```
{
"project_id": "",
"client_email": "",
"private_key": ""
}
```

Please fix this error

Compare the GCP credential-file documentation with the credential handling shown in index.js. Update the documentation so its project field matches the format accepted by the code, then verify that the example and surrounding instructions are consistent.

Written by the indexing model from the issue text.