Update time to first reward

[lrettig]

Right now the guide says:

If you have committed storage and left your app running 24/7, and your network is properly configured, then you should expect your first Smesh reward in about 49 hours, and subsequent rewards every 24 hours.

(https://github.com/spacemeshos/testnet-guide/blob/master/guide/status.md#smesh-rewards-expectancy)

However, this does not factor in the time until the node is fully synced. As the testnet mesh grows in size, we need to consider the time it takes to sync it as well. We've had questions about this on Discord, could we clarify this?

CC @yaelmhoffman @avive