Fix output, bringing it closer to the loop solver

What's reported is not an error but a correction. Only difference to a
loop solver now: In relative mode, the latter outputs absolute
corrections, too.

dune/solvers/solvers/cgsolver.cc

@@ -37,7 +37,10 @@ void CGSolver<MatrixType, VectorType>::solve()
     if (this->verbosity_ == NumProc::FULL)
     {
         std::cout << " iter";
-        std::cout << "          error";
+        if (this->useRelativeError_)
+            std::cout << "     correction ";
+        else
+            std::cout << " abs correction ";
         std::cout << "     rate";
         std::string header;
         if (preconditioner_) {