Fix typo in README.md
Fix typo in README.md

A C++ coding style against *input* parameters is pretty limiting ;-).
mathstuf/sprokit,Kitware/sprokit,Kitware/sprokit,mathstuf/sprokit,linus-sherrill/sprokit,linus-sherrill/sprokit,Kitware/sprokit,mathstuf/sprokit,linus-sherrill/sprokit,Kitware/sprokit,linus-sherrill/sprokit,mathstuf/sprokit