Protect <> in examples

[mglisse]

https://www.boost.org/doc/libs/1_70_0/libs/graph/doc/incremental_components.html The first example, from examples/incremental_components.cpp, was copy-pasted raw, without protecting special characters from HTML, so in particular all templates are invisible.

[anadon]

I'll queue this up for documentation fixes after I'm done with CI.