Fix typo in comment ('contact' -> 'contract') (model-checking#352)

I was grepping for contract and this one didn't match because of the
typo. Minor comment-only change.

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under the terms of the Apache 2.0 and MIT licenses.

Author: patricklam

library/core/src/ptr/const_ptr.rs

@@ -2614,7 +2614,7 @@ mod verify {
     gen_const_byte_arith_harness_for_dyn!(byte_sub, check_const_byte_sub_dyn);
     gen_const_byte_arith_harness_for_dyn!(byte_offset, check_const_byte_offset_dyn);
 
-    // Proof for contact of byte_offset_from to verify unit type
+    // Proof for contract of byte_offset_from to verify unit type
     #[kani::proof_for_contract(<*const ()>::byte_offset_from)]
     pub fn check_const_byte_offset_from_unit() {
         let val: () = ();