Fix a typo, in the Catalog.numPages getter, than prevents shadowing from working correctly

Looking at the blame, it seems that this typo was present even before PR 700 (almost six years ago).
The result of using `'num'`, rather than the *correct* `'numPages'` string, is that the `Catalog.numPages` getter isn't actually being shadowed.

src/core/obj.js

@@ -168,7 +168,7 @@ var Catalog = (function CatalogClosure() {
         'page count in top level pages object is not an integer'
       );
       // shadow the prototype getter
-      return shadow(this, 'num', obj);
+      return shadow(this, 'numPages', obj);
     },
     get destinations() {
       function fetchDestination(dest) {