docs(openstack): put quote around interval value

[MindTooth]

Being a novice, I struggled with TOML errors. Found it to be missing quotes.

Required for all PRs

$ make lint
golangci-lint run
ERRO Running error: unknown linters: 'gocheckcompilerdirectives', run 'golangci-lint help linters' to see the list of supported linters 
make: *** [lint] Error 3

$ make check-deps
+github.com/99designs/go-keychain


The docs/LICENSE_OF_DEPENDENCIES.md file does not contain the expected entries.

Lines prefixed with '+' should be added to LICENSE_OF_DEPENDENCIES.md and '-'
lines should be removed.

Include a link to the appropriate licenses for any additions.
make: *** [check-deps] Error 1

$ make test  # ✅ 

$ make docs  # ✅ 

• Updated associated README.md.
• Wrote appropriate unit tests.
• Pull request title or commits are in conventional commit format

resolves #

[telegraf-tiger]

Thanks so much for the pull request!
🤝 ✒️ Just a reminder that the CLA has not yet been signed, and we'll need it before merging. Please sign the CLA when you get a chance, then post a comment here saying !signed-cla

[MindTooth]

!signed-cla

[powersj]

Thanks