Update mainnet.md
Set mainnet version to 2.0.6
title: MainNet
# Version
-`v2.0.5.stable`
+`v2.0.6.stable`
# Release Version
-https://github.com/algorand/go-algorand/releases/tag/v2.0.5-stable
+https://github.com/algorand/go-algorand/releases/tag/v2.0.6-stable
# Genesis ID
`mainnet-v1.0`
