Fix deprecated warning for interval.decorator.ts

[hkjeffchan]

[DEP0056] DeprecationWarning: The util.isString API is deprecated. Please use typeof arg === "string" instead.

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• [] Tests for the changes have been added (for bug fixes / features)
• [] Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

[DEP0056] DeprecationWarning: The util.isString API is deprecated. Please use typeof arg === "string" instead.

Issue Number: #1757

What is the new behavior?

Fixed DeprecationWarning

Does this PR introduce a breaking change?

• Yes
• No

Other information

[kamilmysliwiec]

LGTM