Corrected README.md
Corrected README.md

Added @ symbol before the InjectExtra in the Optional example.

MaTriXy/saber,stephanenicolas/dart,f2prateek/dart,AbnerLv/dart,MaTriXy/saber,MaTriXy/dart,pakoito/dart,f2prateek/dart,jug6ernaut/saber,bryant1410/dart,bryant1410/dart,jug6ernaut/saber,stephanenicolas/dart,MaTriXy/dart,AbnerLv/dart,pakoito/dart,brunodles/dart,brunodles/dart