Update one of the example in syzygy.gyp_env.README

setting GYP_GENERATORS='ninja-msvs,ninja' leads to some really weird (and hard to debug) errors...: "gyp: Call to '/bin/echo -n ${HOME}/goma' returned exit status 1."

BUG=
R=siggi@chromium.org

Review URL: https://codereview.appspot.com/176010043