docs: fix missing 'of' in contributing guide

Bob Singhrequested to merge
gitlab-community/gitlab-org/api/client-go:docs/fix-missing-of into main

What does this MR do? Fixes a missing word in documentation.

Why was this MR needed? The sentence was grammatically incorrect.

What's the best way to test this MR? Read the updated sentence for correctness.

Relevant issue numbers? N/A

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