Changing headings in ipynb for better readability on readthedocs

Author: michaeldenes

docs/examples/example_add_your_own_kernel.ipynb

@@ -187,7 +187,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "# Create a `Kernel` list\n",
+    "## Create a `Kernel` list\n",
     "The virtual plastic particles will experience any behaviour that we apply to it. `plasticparcels` contains a few prebuilt behaviours (see [here](https://plastic.oceanparcels.org/en/latest/physicskernels.html) for more details). To use these behaviours, we must include a list of `Kernel` objects. Each `Kernel` describes some physics behaviour to be applied. To include the prebuilt kernels, we use the `constructors.create_kernel()` method."
    ]
   },
@@ -324,7 +324,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "# Implementing your own kernel\n",
+    "## Implementing your own kernel\n",
     "In the case you want to include your own kernel or modify an existing kernel, you simply need to define your kernel and include it in the list of kernels. More details on how this works can be found [here](https://docs.oceanparcels.org/en/latest/examples/tutorial_kernelloop.html).\n",
     "\n",
     "Note: You must include the kernel before the error checking kernels, which are currently the last five kernels."