docs: fix a small typo in injectable decorator description

[timdeschryver]

PR Checklist

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• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

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• Bugfix
• Feature
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What is the current behavior?

Issue Number: N/A

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Does this PR introduce a breaking change?

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• No

Other information

[brandonroberts]

Caretaker note: This is a docs only change and should not impact g3. Please run presubmit.

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