DOC: Update URL for the <img> element

Author: mgeier

doc/markdown-cells.ipynb

@@ -330,7 +330,7 @@
     "\n",
     "If you want to control the size of the included image,\n",
     "you can use the HTML\n",
-    "[\\<img\\>](https://www.w3.org/TR/html52/semantics-embedded-content.html#the-img-element)\n",
+    "[\\<img\\>](https://html.spec.whatwg.org/multipage/embedded-content.html#the-img-element)\n",
     "element with the `width` attribute like this:\n",
     "\n",
     "```html\n",