message: Update proof README
diff: # Language formalisation in Lean
  -This is incomplete, and mainly indended as an explorative tool to guide the Rust
  -implementation. It would be very nice to provide more guarentees in the future
  -that opur Rust implementation matches this formalisation, but that is out of
  -scope for now!
  +This is incomplete, and mainly intended as an explorative tool to guide the Rust
  +implementation. Many theorems and lemmas remain to be been proven. It would be
  +very nice to provide more guarentees in the future that our Rust implementation
  +matches this formalisation, but that is out of scope for now!
  To work with the formalisations, you will need to [install Lean 3.3][install_lean].
  Then run the following commands in the shell:
  