Fix comment

src/myblockproblem.hh

@@ -107,7 +107,7 @@ class MyBlockProblem<ConvexProblemTypeTEMPLATE>::IterateObject {
         if (j == m)
           localA = &(*it); // localA = &A[m][m]
         else
-          it->mmv(u[j], localb); // localb += A[m][j] * u[j]
+          it->mmv(u[j], localb); // localb -= A[m][j] * u[j]
       }
       assert(localA != NULL);