Fix typo (#380)

Author: SUPERCILEX

src/subtyping.md

@@ -252,7 +252,7 @@ type of the referent is shared knowledge, which is why adjusting that type in on
 one place (the reference) can lead to issues. But if you shrink down a reference's
 lifetime when you hand it to someone, that lifetime information isn't shared in
 any way. There are now two independent references with independent lifetimes.
-There's no way to mess with original reference's lifetime using the other one.
+There's no way to mess with the original reference's lifetime using the other one.
 
 Or rather, the only way to mess with someone's lifetime is to build a meowing dog.
 But as soon as you try to build a meowing dog, the lifetime should be wrapped up