doc: synchronize argument names for appendFile()

[Trott]

The documentation used file for the first argument to appendFile()
functions. However, the code and (more importantly) thrown errors
referred to it as path. The latter is especially important because
context is not provided. So you're looking for a function that takes
path but that string doesn't appear in your code or in the
documentation. It's not until the end user looks at the source code of
Node.js that they can figure out what's going on. This is why it is
important that the names of variables in the documentation match that in
the code. If we want to change this to file, then that's OK, but we
need to do it in the source code and error messages too, not just in the
docs. Changing the docs is the smallest change to synchronize everything
so that's what this change does.

Checklist

• documentation is changed or added
• commit message follows commit guidelines

[Trott]

Why this matters:

$ node
> fs.appendFile(undefined, 'foo', () => {})
TypeError [ERR_INVALID_ARG_TYPE]: The "path" argument must be one of type string, Buffer, or URL. Received type undefined
    at Object.fs.open (fs.js:523:3)
    at Object.fs.writeFile (fs.js:1253:6)
    at Object.fs.appendFile (fs.js:1308:6)
>

The error message refers to it as the "path" argument. So the documentation should do the same or else things are puzzling for the user until they look at the source code for Node.js.

[Trott]

Lite CI: https://ci.nodejs.org/job/node-test-pull-request-lite/653/

[vsemozhetbyt]

Node.js Collaborators, please, add 👍 here if you approve fast-tracking.

[Trott]

Landed in a00f76c