Update doc/cprover-manual/requires-and-ensures-clauses.md

Co-authored-by: Saswat Padhi <[email protected]>

Author: feliperodri

doc/cprover-manual/requires-and-ensures-clauses.md

@@ -41,7 +41,7 @@ To illustrate these two perspectives, consider the following implementation of t
 
 ```c
 int sum(uint32_t* a, uint32_t* b, uint32_t* out)
-/* Precondition */
+/* Preconditions */
 __CPROVER_requires(__CPROVER_is_fresh(a, sizeof(*a)))
 __CPROVER_requires(__CPROVER_is_fresh(b, sizeof(*b)))
 __CPROVER_requires(__CPROVER_is_fresh(out, sizeof(*out)))