Harmonize the headers of each tutorial file + fix some typos

Author: luseverin

doc/guide/Guide_Introduction.ipynb

@@ -61,12 +61,6 @@
     "    Cambridge University Press, United Kingdom and New York, NY,\n",
     "    USA., 2014."
    ]
-  },
-  {
-   "cell_type": "markdown",
-   "id": "503ec4a7",
-   "metadata": {},
-   "source": []
   }
  ],
  "metadata": {

doc/tutorial/climada_engine_Impact.ipynb

@@ -85,7 +85,7 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "## Read discount rates of an Excel file\n",
+    "## Read discount rates of an Excel file\n",
     "\n",
     "Discount rates defined in an excel file following the template provided in sheet `discount` of `climada_python/climada/data/system/entity_template.xlsx` can be ingested directly using the method `from_excel()`."
    ]