Fix Solver::Options::callbacks type in documentation

Change-Id: I66374bd1b45670a1bcd5924287d855d9c44826e2

docs/source/nnls_solving.rst

diff --git a/docs/source/nnls_solving.rst b/docs/source/nnls_solving.rst
index 226e5f6..184b94b 100644
--- a/docs/source/nnls_solving.rst
+++ b/docs/source/nnls_solving.rst
@@ -2072,7 +2072,7 @@
    should not expect to look at the parameter blocks and interpret
    their values.
 
-.. member:: std::vector<IterationCallback> Solver::Options::callbacks
+.. member:: std::vector<IterationCallback*> Solver::Options::callbacks
 
    Default: ``empty``