Fix instructions to get rust-analyzer to work

[wooster0]

This fixed it for me.

[JohnTitor]

Duplicate of #1211, cc @camelid could we merge your PR now?

[wooster0]

Oh, my bad.
I agree, I think it'd be nice to have this fixed because the error that rust-analyzer gives for this is not very helpful and probably confuses other contributors too.

[ERROR rust_analyzer::reload] failed to switch workspaces: rust-analyzer failed to load workspace: Failed to read Cargo metadata from Cargo.toml file /home/roost/Code/rust/Cargo.toml, cargo 1.57.0-nightly (d56b42c54 2021-09-27): Failed to run `cargo metadata --manifest-path /home/roost/Code/rust/Cargo.toml`: `cargo metadata` exited with an error: error: failed to load manifest for workspace member `/home/roost/Code/rust/src/tools/rust-installer`

Caused by:
  failed to read `/home/roost/Code/rust/src/tools/rust-installer/Cargo.toml`

Caused by:
  No such file or directory (os error 2)