simplify wording

Author: RalfJung

library/std/src/keyword_docs.rs

@@ -2147,7 +2147,7 @@ mod type_keyword {}
 /// (like `use_indexable` does), but the *implementation* of `get_unchecked` has no proof obligation
 /// to contend with. Note that unlike normal `unsafe fn`, an `unsafe fn` in a trait implementation
 /// does not get to just pick an arbitrary safety contract! It *has* to use the safety contract
-/// defined by the trait (or a stronger contract, i.e., weaker preconditions).
+/// defined by the trait (or one with weaker preconditions).
 ///
 /// Of course, the implementation may choose to call other unsafe operations, and then it needs an
 /// `unsafe` *block* to indicate it discharged the proof obligations of its callees. For