Add warning, correct minor typo

add warning: developer install is needed to run this tuto as component is not included in latest release,

Author: margauxmouchene

transport-length_hillslope_diffuser/TLHDiff_tutorial.ipynb

@@ -25,6 +25,14 @@
     "<hr>"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "# WARNING\n",
+    "**Some components used in this tutorial are not included in the latest release of Landlab. You need to do the [developer install](https://github.com/landlab/landlab/wiki/Installing-Landlab-from-source-code-(%22developer-install%22) to use them and run this tutorial.**"
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},