file upload example is broken

[aoberoi]

• I've read and understood the Contributing guidelines and have done my best effort to follow them.
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• I've searched for any related issues and avoided creating a duplicate issue.

Description

The example found at https://slackapi.github.io/node-slack-sdk/basic_usage#uploading-a-file does not work. "fs.createReadStream(filePath) should be fs.createReadStream(fileName)" (thanks @mhahn)

Reproducible in:

node slack sdk version: *
OS version(s): *
Device(s): *

Steps to reproduce:

1. Try the example code, witness failure.

Expected result:

a file to be uploaded successfully

Actual result:

runtime failure

[clavin]

I believe the description given isn't fully accurate: fs.createReadStream(filePath) should indeed be itself, but filePath should instead point to a file rather than a directory.

The file name and path are different (or at least can be, and that should be shown off) so the change should definitely not remove the filePath variable.

Just clarifying.