Error in FunctorImageFilter.rst example

Summary

A small error in doc spotted during discussion on #1960 (closed).

Copyright

The copyright owner is CNES and has signed the ORFEO ToolBox Contributor License Agreement.

---

Check before merging:

• All discussions are resolved
• At least 2 votes from core developers, no vote.
• The feature branch is (reasonably) up-to-date with the base branch
• Dashboard is green
• Copyright owner has signed the ORFEO ToolBox Contributor License Agreement
• Optionally, run git diff develop... -U0 --no-color | clang-format-diff.py -p1 -i on latest changes and commit

Documentation/Cookbook/rst/C++/FunctorImageFilter.rst

@@ -142,7 +142,7 @@ This is strictly equivalent to:
 
 .. code-block:: cpp
 
-    void myFreeFunction(const itk::VariableLengthVector<double> & out ,
+    void myFreeFunction(itk::VariableLengthVector<double> & out ,
                         unsigned char a, 
                         const std::complex<float> & b,
                         const itk::VariableLengthVector<short> &c,