The search engine uses stemming, which reduces words to their root form. A search for "origins" stems back to "origin", so search results may include other words with that root, such as "origin", "original", "originals", and "origins". There is currently no way to disable that behavior, though it is useful in the majority of searches.
In this case here, it's preferable: you can just search for "Assassins Creed Origins". Stemming converts each word back to its root, so this becomes a search for "assassin creed origin", and it will show you all the posts that have all those roots in them. Thus, it will show you discussions about the game regardless of whether the author used "Assassin's", "Assassins", or just "Assassin".