Rephrase text to make it clear setup process is trust-minimized

[billygarrison]

https://github.com/jamesob/coldcore/blob/3a8395cb185ae36c4195a506a4df5cdfdbf045ac/coldcore#L2141

The use of the word us here could be misunderstood to mean the chain rescan is happening on some "official" coldcore servers.