turn link to RFC 1751 into a markdown link

Author: nivkner

src/librustc_borrowck/borrowck/README.md

@@ -781,7 +781,7 @@ the base path, it will still be considered freezable.
 
 
 
-**FIXME(https://github.com/rust-lang/rfcs/issues/1751)
+**FIXME [RFC 1751](https://github.com/rust-lang/rfcs/issues/1751)
 Restrictions against mutating the base pointer.**
 When an `&mut` pointer is frozen or claimed, we currently pass along the
 restriction against MUTATE to the base pointer. I do not believe this