Fix tutorial generation warnings

Fix some warnings when generating tutorials using `sphinx-build`

[new output (117 lines)](https://gist.github.com/VictorLamoine/b96bd9bcae1e8f6eb3bf)
[old output (146 lines)](https://gist.github.com/VictorLamoine/b96bd9bcae1e8f6eb3bf#file-old-output)
It looks like `sphinx-build` can't parse `lisp`:
```
~/pcl/doc/advanced/content/pcl_style_guide.rst:339: WARNING: Pygments lexer name u'lisp' is not known
```
So I replaced it with `cpp` to shut the warning down.