Update off-canvas menu icon to be visible on Android 2.3 devices

As detailed on CanIUse:
>Android 2.3 default browser doesn't seem to appreciate 0px values. e.g. -webkit-box-shadow: 5px 1px 0px #f04e29; doesn't work, but -webkit-box-shadow: 5px 1px 1px #f04e29 does!
http://caniuse.com/#search=box-shadow

This is the case on Android 2.3 browsers (i.e. a tested Samsung Galaxy S2). Modifying the 0 value to 1px only for the -webkit prefixed box-shadow solves the issue without causing visual degradation, ensuring the icon appears on all  modern devices.