DOC: fix broken link

Author: mgeier

doc/usage.ipynb

@@ -316,7 +316,7 @@
    "source": [
     "## HTML Themes\n",
     "\n",
-    "The `nbsphinx` extension does *not* provide its own theme, you can use any of the available themes or [create a custom one](https://www.sphinx-doc.org/en/master/development/theming.html#creating-themes), if you feel like it.\n",
+    "The `nbsphinx` extension does *not* provide its own theme, you can use any of the available themes or [create a custom one](https://www.sphinx-doc.org/en/master/development/html_themes/index.html#creating-themes), if you feel like it.\n",
     "\n",
     "The following (incomplete) list of themes contains up to three links for each theme:\n",
     "\n",