fix via mdinger

repr-rust.md

@@ -109,7 +109,7 @@ struct FooRepr {
 
 And indeed this is approximately how it would be laid out in general
 (modulo the size and position of `tag`). However there are several cases where
-such a representation is ineffiecient. The classic case of this is Rust's
+such a representation is inefficient. The classic case of this is Rust's
 "null pointer optimization". Given a pointer that is known to not be null
 (e.g. `&u32`), an enum can *store* a discriminant bit *inside* the pointer
 by using null as a special value. The net result is that