[System] Fix parsing of URI schemes with digits

Initializing an Uri previously throwed an exception when the URI scheme contained a digit (e.g. net.p2p).
The loop was missing a digit check, similar to https://github.com/mono/mono/blob/1f1dc988ad7b165603a1175bf0a92156c4372c43/mcs/class/System/System/Uri.cs#L767

I noticed this after PR #1168 by @esdrubal was merged.