Fix $PATH ordering in README

The README currently suggests to set your $PATH like so:

export PATH="$HOME/.cabal/bin:$HOME/.ghcup/bin:$PATH"

However, this means the system-installed cabal overrides the ghcup installed cabal. This recently bit me. Instead, I believe the $PATH should be set like so:

export PATH="$HOME/.ghcup/bin:$HOME/.cabal/bin:$PATH"