Example: allow the use of SIFT and SURF

When running `cpp-example-matching_to_many_images` the default help text suggests using SURF feature detector and SURF feature descriptor. However, these are not available any more.

Following the instructions in the help text results in a runtime error.
There are two ways of solving this. Either include the nonfree modules of
OpenCV, or change the help text to use only free Feature Detectors.
I chose the first option.