Nit: fix typo

Author: nikomatsakis

src/librustc_mir/transform/nll/region_infer.rs

@@ -92,7 +92,7 @@ impl Region {
 #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub struct Constraint {
     // NB. The ordering here is not significant for correctness, but
-    // it is for conenience. Before we dump the constraints in the
+    // it is for convenience. Before we dump the constraints in the
     // debugging logs, we sort them, and we'd like the "super region"
     // to be first, etc. (In particular, span should remain last.)