[doxygen] Fix PETSc links.

Documentation/ProjectFile/prj/linear_solvers/linear_solver/petsc/t_prefix.md

 Sets the options prefix for this PETSc linear solver.
 
-Cf. the PETSc [KSPSetOptionsPrefix documentation](http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/KSP/KSPSetOptionsPrefix.html).
+Cf. the PETSc [KSPSetOptionsPrefix documentation](https://petsc.org/release/manualpages/KSP/KSPSetOptionsPrefix/).

Documentation/ProjectFile/prj/nonlinear_solvers/nonlinear_solver/t_prefix.md

 Sets the options prefix for the PETSc SNES solver.
 
 See the PETSc
-[SNESSetOptionsPrefix](https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/SNES/SNESSetOptionsPrefix.html)
+[SNESSetOptionsPrefix](https://petsc.org/release/manualpages/SNES/SNESSetOptionsPrefix/)
 documentation for details.

MathLib/LinAlg/PETSc/PETScLinearSolver.h

@@ -42,7 +42,7 @@ public:
                        that owns an instance of this class.
         \param petsc_options PETSc options string which is passed to PETSc lib
         and inserted in the PETSc option database (see
-        https://petsc.org/release/docs/manualpages/Sys/PetscOptionsInsertString.html).
+        https://petsc.org/release/manualpages/Sys/PetscOptionsInsertString/).
     */
     PETScLinearSolver(std::string const& prefix,
                       std::string const& petsc_options);
@@ -69,4 +69,4 @@ private:
     double elapsed_ctime_ = 0.0;  ///< Clock time
 };
 
-}  // end namespace
+}  // namespace MathLib