Fix typo

Co-authored-by: Yuki Okushi <jtitor@2k36.org>

src/phantom-data.md

@@ -106,7 +106,7 @@ that that `Vec<T>` _owns_ values of type `T` (more precisely: may use values of
 in its `Drop` implementation), and Rust will thus not allow them to _dangle_ should a
 `Vec<T>` be dropped.
 
-**Adding an extra `_owns_T: PhantomData<T>` field is thus _superflous_ and accomplishes nothing**.
+**Adding an extra `_owns_T: PhantomData<T>` field is thus _superfluous_ and accomplishes nothing**.
 
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