s/to to/to/

As <iostream> is not included in json.hpp any more, all code examples need to include <iostream> now.

Maybe the simple approach is working now - at least it is in https://github.com/taocpp/PEGTL/blob/master/.travis.yml

Removed deprecated documentation of the comparison operator.

The documentation mismatched the actual code.

To have nothrow-copy-constructible exceptions, we inherit from
std::runtime_error which can cope with arbitrary-length error messages.
Intermediate strings are built with static functions and then passed to
the actual constructor.