Fixed screenshots in sample READMEs

I successfully ran to update screenshots in samples READMEs:

    find . -name "README.md" -print | xargs sed -i 's/\[screenshot\](https:\/\/raw.github.com\/GoogleChrome\/chrome-app-samples\/master\//\[screenshot\](\/samples\//g'

BUG=#310
TBR=@mangini