CONTRIBUTING: fix a small typo (#388199)

CONTRIBUTING.md

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 For the committer to judge your intention, it's best to explain why you've made your change.
 This does not apply to trivial changes like version updates because the intention is obvious (though linking the changelog is appreciated).
-For any more nuanced changed or even major version upgrades, it helps if you explain the background behind your change a bit.
+For any more nuanced changes or even major version upgrades, it helps if you explain the background behind your change a bit.
 E.g. if you're adding a package, explain what it is and why it should be in Nixpkgs.
 This goes hand in hand with [Writing good commit messages](#writing-good-commit-messages).