Fix dead links in cprover manual

Author: danpoe

doc/cprover-manual/api.md

@@ -18,7 +18,7 @@ void assert(_Bool assertion);
 The function **\_\_CPROVER\_assume** adds an expression as a constraint
 to the program. If the expression evaluates to false, the execution
 aborts without failure. More detail on the use of assumptions is in the
-section on [Assumptions](./modeling-assumptions.md).
+section on [Assumptions](http://www.cprover.org/cprover-manual/modeling/assumptions/).
 
 #### \_\_CPROVER\_r_ok, \_\_CPROVER\_w_ok
 

doc/cprover-manual/cbmc-assertions.md

@@ -33,7 +33,7 @@ properties together with the condition.
 
 The assertion language of the CPROVER tools is identical to the language
 used for expressions.  Note that
-[nondeterminism](./modeling-nondeterminism.md) can be exploited in order
+[nondeterminism](http://www.cprover.org/cprover-manual/modeling/nondeterminism/) can be exploited in order
 to check a range of choices.  As an example, the following code fragment
 asserts that **all** elements of the array are zero: