fix(page): fix page.screenshot method (#4786)

This patch fixes page.screenshot. so that it throws correct error message when options.clip.height===0

Author: himaozi

Diff for: lib/Page.js

@@ -879,7 +879,7 @@ class Page extends EventEmitter {
       assert(typeof options.clip.width === 'number', 'Expected options.clip.width to be a number but found ' + (typeof options.clip.width));
       assert(typeof options.clip.height === 'number', 'Expected options.clip.height to be a number but found ' + (typeof options.clip.height));
       assert(options.clip.width !== 0, 'Expected options.clip.width not to be 0.');
-      assert(options.clip.height !== 0, 'Expected options.clip.width not to be 0.');
+      assert(options.clip.height !== 0, 'Expected options.clip.height not to be 0.');
     }
     return this._screenshotTaskQueue.postTask(this._screenshotTask.bind(this, screenshotType, options));
   }