Remove reference to old repo name

[jdahm]

From a user

A question about Serialbox.... When I go to https://github.com/GridTools/serialbox, it points me to build/install documentation that says I need to git clone https://github.com/eth-cscs/serialbox2.git . Is that correct? Which repository should I get and build Serialbox from?

This is confusing and should be corrected.