Update GradientChecker docs

* Use GradientChecker::ProbeResults instead of GradientCheckResults
* Add missing parenthesis in if statement

Change-Id: I900608eb806753ec8f42c09d395fa3b93f012e76

docs/source/nnls_modeling.rst

diff --git a/docs/source/nnls_modeling.rst b/docs/source/nnls_modeling.rst
index 9d33b51..ae34e6b 100644
--- a/docs/source/nnls_modeling.rst
+++ b/docs/source/nnls_modeling.rst
@@ -915,8 +915,8 @@
        GradientChecker gradient_checker(my_cost_function,
                                         manifolds,
                                         numeric_diff_options);
-       GradientCheckResults results;
-       if (!gradient_checker.Probe(parameter_blocks.data(), 1e-9, &results) {
+       GradientChecker::ProbeResults results;
+       if (!gradient_checker.Probe(parameter_blocks.data(), 1e-9, &results)) {
          LOG(ERROR) << "An error has occurred:\n" << results.error_log;
        }