Fixed typo in getting-started.md

[alex-drocks]

Just added missing "a" in the first sentence.

Changes

First sentence did not include the "a". Please see updated file git comparison.

Testing

No test needed, single char added in documentation first page. Merge created in Github with merge checks.

Docs

Yes. Doc updated. Only 1 char added because it was missing in the sentence.

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[jasikpark]

LGTM! Thanks for the typofix :)