fix typo in primitive reference docs

Author: ericseppanen

library/core/src/primitive_docs.rs

@@ -1647,7 +1647,7 @@ mod prim_usize {}
 /// For the other direction, things are more complicated: when unsafe code passes arguments
 /// to safe functions or returns values from safe functions, they generally must *at least*
 /// not violate these invariants. The full requirements are stronger, as the reference generally
-/// must point to data that is safe to use at type `T`.
+/// must point to data that is safe to use as type `T`.
 ///
 /// It is not decided yet whether unsafe code may violate these invariants temporarily on internal
 /// data. As a consequence, unsafe code which violates these invariants temporarily on internal data