fix typos

Author: thanhnguyen-aws

doc/src/challenges/0017-slice.md

@@ -63,7 +63,7 @@ The safety of the following functions in (library/core/src/slice/mod.rs):
 
 The verification must be unbounded---it must hold for slices of arbitrary length.
 
-The verification must be hold for generic type `T` (no monomorphization).
+The verification must hold for generic type `T` (no monomorphization).
 
 ### List of UBs
 

doc/src/challenges/0018-slice-iter-pt1.md

@@ -67,7 +67,7 @@ Write and prove the contract for the safety of the following functions:
 
 The verification must be unbounded---it must hold for slices of arbitrary length.
 
-The verification must be hold for generic type `T` (no monomorphization).
+The verification must hold for generic type `T` (no monomorphization).
 
 ### List of UBs
 

doc/src/challenges/0019-slice-iter-pt2.md

@@ -69,7 +69,7 @@ Write and prove the contract for the safety of the following functions:
 
 The verification must be unbounded---it must hold for slices of arbitrary length.
 
-The verification must be hold for generic type `T` (no monomorphization).
+The verification must hold for generic type `T` (no monomorphization).
 
 ### List of UBs