Correct path in use_stub_resolver documentation (#177)

d1c2ec39f2a238033a64345f31829dc6b1c0b6b9 added use_stub_resolver as a
parameter but there's a mismatch between the code and documentation.
While in practice /var/run is a symlink to /run and the mismatch doesn't
really matter, it's better to have them match to avoid confusion.