Eliminate redundancy in ES6 AssignmentExpression

Now that MemberExpression implements Pattern (see dfb7d88), it is redundant to say that an assignment's left-hand side can be either a pattern or a member expression.

es6.md

@@ -120,7 +120,7 @@ Spread expression, e.g., `[head, ...iter, tail]`, `f(head, ...iter, ...tail)`.
 
 ```js
 extend interface AssignmentExpression {
-    left: Pattern | MemberExpression;
+    left: Pattern;
 }
 ```