no longer reference header-footer.ipynb as this file is now a .md

Author: parmentelat

doc/customize.md

@@ -172,10 +172,10 @@ is still responsible for cosmetic styling:
     }
 
 You can see some examples using these options at
-`RISE/examples/overlay.ipynb` and `RISE/examples/header-footer.ipynb`,
+`RISE/examples/overlay.ipynb` and `RISE/examples/header-footer.md`,
 or in binder respectively
 [![](https://mybinder.org/badge.svg)](https://mybinder.org/v2/gh/damianavila/RISE/master?filepath=examples%2Foverlay.ipynb)
-[![](https://mybinder.org/badge.svg)](https://mybinder.org/v2/gh/damianavila/RISE/master?filepath=examples%2Fheader-footer.ipynb)
+[![](https://mybinder.org/badge.svg)](https://mybinder.org/v2/gh/damianavila/RISE/master?filepath=examples%2Fheader-footer.md)
 
 
 ### Enable a right scroll bar

examples/overlay.ipynb

@@ -116,7 +116,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "The present notebook demonstrates defining `overlay`, [this other one](header-footer.ipynb) shows how to \n",
+    "The present notebook demonstrates defining `overlay`, [this other one](header-footer.md) shows how to \n",
     "use `header`, `footer` and `backimage`."
    ]
   }