Fixed typo in GenerateSW JSDocs

[Spiderpig86]

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R: @jeffposnick @philipwalton

Fixes #issue number

Description of what's changed/fixed.

Fixed typo I found while going through reference docs for GenerateSW. Since no logic was changed, I don't think an issue needs to exist prior to PR. 🙂

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[Spiderpig86]

@googlebot I signed it!

[jeffposnick]

Thank you!