docs: update "building with crane" example in README.md

[dvtfl]

Since crane v0.19.0, (ipetkov/crane@6f7504a), the crane.lib no longer exists, but still mentioned in readme.md.

fenix/README.md

Line 466 in fa3610f

craneLib = crane.lib.${system}.overrideToolchain

$ nix eval
warning: input 'crane' has an override for a non-existent input 'flake-utils'
warning: input 'crane' has an override for a non-existent input 'nixpkgs'
error: attribute 'lib' missing
       at /nix/store/bqx681q21lm8ynqbd5r0jc69y03dxgfh-source/flake.nix:22:24:
           21|           let
           22|             craneLib = crane.lib.${system}.overrideToolchain
             |                        ^
           23|               fenix.packages.${system}.minimal.toolchain;