Fix _.size doc typo. [ci skip] [closes #394]

dist/lodash.underscore.js

@@ -2666,7 +2666,7 @@
    * // => 3
    *
    * _.size('pebbles');
-   * // => 5
+   * // => 7
    */
   function size(collection) {
     var length = collection ? collection.length : 0;