[21.01] Fix typo in upstream_mod_zip admin docs

[nsoranzo]

What did you do?

• Fix typo in upstream_mod_zip admin docs

Why did you make this change?

There was a typo introduced in #10919 .

How to test the changes?

(select the most appropriate option; if the latter, provide steps for testing below)

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

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