Fix typos in the documentation for EvaluationCallback

https: //github.com/ceres-solver/ceres-solver/issues/953
Change-Id: I6631c152f794c9492e527ceff11d7d9e598b92f8

docs/source/nnls_modeling.rst

diff --git a/docs/source/nnls_modeling.rst b/docs/source/nnls_modeling.rst
index 86472c2..4b558dd 100644
--- a/docs/source/nnls_modeling.rst
+++ b/docs/source/nnls_modeling.rst
@@ -2433,7 +2433,7 @@
 
       class EvaluationCallback {
        public:
-        virtual ~EvaluationCallback() = default;
+        virtual ~EvaluationCallback();
         virtual void PrepareForEvaluation(bool evaluate_jacobians,
                                           bool new_evaluation_point) = 0;
       };