Fix typo

Author: aturon

text/0000-cmp-ops-reform.md

@@ -313,10 +313,12 @@ In addition, this RFC proposes another trait, `IndexSet`, that is used for `expr
 ```rust
 pub trait IndexSet<Idx> {
     type Val;
-    fn index_mut<'a>(&'a mut self, index: Idx, val: Val);
+    fn index_set<'a>(&'a mut self, index: Idx, val: Val);
 }
 ```
 
+(This idea is borrowed from [@sfackler's earlier RFC](https://github.com/rust-lang/rfcs/pull/159/files).)
+
 The motivation for this trait is cases like `map["key"] = val`, which
 should correspond to an *insertion* rather than a mutable lookup. With
 today's setup, that expression would result in a panic if "key" was