Fix lint errors in oauth package

[seratch]

Summary

The CI builds for the latest revision fails as the latest lslint detects two things in the new code.

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[seratch]

Interestingly, Node 14's linter says a different thing. I will update the code to satisfy both (perhaps, the comment to ignore these)

ERROR: /home/runner/work/node-slack-sdk/node-slack-sdk/packages/oauth/src/stores/file-store.ts:93:64 - Unexpected parentheses around single function argument having a body with no curly braces.
ERROR: /home/runner/work/node-slack-sdk/node-slack-sdk/packages/oauth/src/stores/file-store.ts:98:25 - Unexpected parentheses around single function argument having a body with no curly braces.

[seratch]

This is not ideal but I've added comment flags to satisfy all the supported Node runtimes. Let me merge this now.