Added note at top of colab to remind people to enable a gpu backend. (#1628)

# Description

as above. in case your grandma wants to use it.

# Checklist:

- [x] I have changed the base branch to `dev`
- [x] I have performed a self-review of my own code
- [x] I have commented my code in hard-to-understand areas
- [ ] I have made corresponding changes to the documentation

Web_based_UI_for_Stable_Diffusion_colab.ipynb

@@ -265,7 +265,9 @@
     {
       "cell_type": "markdown",
       "source": [
-        "# Config options for Colab instance"
+        "# Config options for Colab instance\n",
+        "> Before running, make sure GPU backend is enabled. (Unless you plan on generating with Stable Horde)\n",
+        ">> Runtime -> Change runtime type -> Hardware Accelerator -> GPU (Make sure to  save)"
       ],
       "metadata": {
         "id": "iegma7yteERV"
@@ -549,4 +551,4 @@
       "outputs": []
     }
   ]
-}
+}