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"

README.md

@@ -50,7 +50,7 @@ E.g.:
 Then adjust your `PATH` in `~/.bashrc` (or similar, depending on your shell) like so, for example:
 
 ```sh
-export PATH="$HOME/.cabal/bin:$HOME/.ghcup/bin:$PATH"
+export PATH="$HOME/.ghcup/bin:$HOME/.cabal/bin:$PATH"
 ```
 
 Security aware users may want to use the files from the [release page](https://gitlab.haskell.org/haskell/ghcup/tags/)