Update index.md - typo fix

[johnfmorton]

Fixes #SOME_ISSUE_NUMBER

Changes proposed in this pull request:

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:white_check_mark: Preview (static) for commit 8f5f363f982d941cacfd2466939ef645bd00d5db available at https://pr-7849-static-dot-dcc-staging.uc.r.appspot.com/.

The following pages likely changed with this PR:

• https://pr-7849-static-dot-dcc-staging.uc.r.appspot.com/docs/workbox/modules/workbox-recipes/