Minor updates to units notebook

notebooks/astropy-units-completed.ipynb

@@ -86,13 +86,13 @@
    "id": "93658d4f",
    "metadata": {},
    "source": [
-    "[astropy.units]: https://docs.astropy.org/en/stable/units/index.html\n",
-    "[plasmapy.particles]: ../../particles/index.rst\n",
-    "[plasmapy.formulary]: ../../formulary/index.rst\n",
+    "[`astropy.units`]: https://docs.astropy.org/en/stable/units/index.html\n",
+    "[`plasmapy.particles`]: ../../particles/index.rst\n",
+    "[`plasmapy.formulary`]: ../../formulary/index.rst\n",
     "\n",
-    "Representing a physical quantity as a number has risks. We might unknowingly perform operations with different units, like `time_in_seconds + time_in_hours`. We might even accidentally perform operations with physically incompatible units, like `length + time`, without catching our mistake. We can avoid these problems by using a units package.\n",
+    "Representing a physical quantity as a number has risks. We might unknowingly perform operations with different units, like `time_in_seconds + time_in_hours`. We might even accidentally perform operations with physically incompatible units, like `length + time`, without catching our mistake. Unit conversion errors can be costly mistakes, such as the loss of spacecraft like the [Mars Climate Orbiter](https://science.nasa.gov/mission/mars-climate-orbiter).\n",
     "\n",
-    "This notebook introduces [astropy.units] with an emphasis on the functionality needed to work with [plasmapy.particles] and [plasmapy.formulary]. We typically import this subpackage as `u`."
+    "We can avoid these problems by using a units package. This notebook introduces [`astropy.units`] with an emphasis on the functionality needed to work with [`plasmapy.particles`] and [`plasmapy.formulary`]. We typically import [`astropy.units`] subpackage as `u`."
    ]
   },
   {