Fix silly mix{o=e}r typo in Trait

* lib/Trait.js (_validateMixin): Change docblock and error message to
    resolve typo.

lib/Trait.js

@@ -251,7 +251,7 @@ Trait.extend = function( /* ... */ )
  * into something of type S; a `TypeError` will be thrown if this contract
  * is violated.
  *
- * @param  {Class}  base  mixor (target of mixin)
+ * @param  {Class}  base  mixer (target of mixin)
  * @param  {Trait}  T     mixee (trait being mixed in)
  *
  * @return  {undefined}
@@ -272,7 +272,7 @@ function _validateMixin( base, T )
     {
         throw TypeError(
             "Cannot mix trait " + T.toString() + " into " + base.toString() +
-                "; mixor must be of type " + T.__extbase.toString()
+                "; mixer must be of type " + T.__extbase.toString()
         );
     }
 }