Digraph warning

Update json.hpp

Remove duplicate word

mistake

s/to to/to/

To fix compilation issue for intel OSX compiler

o To prevent the compilation issue on OSX with the intel compiler suite.  The
error was found with icpc version 15.0.3.187 where the "__clang_version__" was
not defined correctly, while "__clang__" was.

The original version can trigger a digraph-related warning in VS2015. Is this namespace construct a typo or something deliberate?

massage -> message

Fix "not constraint" grammar in docs