fix (docs): change duration to a string

[foobarbecue]

Running this example as written gives json: cannot unmarshal number into Go struct field Backoff.workflow.spec.templates.retryStrategy.backoff.duration of type string

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this is a chore.
• The title of the PR is (a) conventional, (b) states what changed, and (c) suffixes the related issues number. E.g. "fix(controller): Updates such and such. Fixes #1234".
• I have written unit and/or e2e tests for my change. PRs without these are unlike to be merged.
• Optional. I've added My organization is added to the README.
• I've signed the CLA and required builds are green.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@7bf0819). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #2380   +/-   ##
=========================================
  Coverage          ?   13.09%           
=========================================
  Files             ?       72           
  Lines             ?    25080           
  Branches          ?        0           
=========================================
  Hits              ?     3283           
  Misses            ?    21380           
  Partials          ?      417

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7bf0819...91eb4f9. Read the comment docs.