Update const eval for dereferencing raw pointers

From my understanding of https://github.com/rust-lang/rust/pull/89551, constant expressions can now dereference non-mutable raw pointers.  I believe [this line](https://github.com/rust-lang/reference/blob/06f9e61931bcf58b91dfe6c924057e42ce273ee1/src/const_eval.md?plain=1#L42) needs updating.