Fix mistake in ch.7

This is the `revocable` function that returns *proxy* and *revoke*.

es6 & beyond/ch7.md

@@ -515,7 +515,7 @@ pobj.a;
 
 A revocable proxy is created with `Proxy.revocable(..)`, which is a regular function, not a constructor like `Proxy(..)`. Otherwise, it takes the same two arguments: *target* and *handlers*.
 
-The return value of `Proxy.revoke(..)` is not the proxy itself as with `new Proxy(..)`. Instead, it's an object with two properties: *proxy* and *revoke* -- we used object destructuring (see "Destructuring" in Chapter 2) to assign these properties to `pobj` and `prevoke()` variables, respectively.
+The return value of `Proxy.revocable(..)` is not the proxy itself as with `new Proxy(..)`. Instead, it's an object with two properties: *proxy* and *revoke* -- we used object destructuring (see "Destructuring" in Chapter 2) to assign these properties to `pobj` and `prevoke()` variables, respectively.
 
 Once a revocable proxy is revoked, any attempts to access it (trigger any of its traps) will throw a `TypeError`.