MNT: Fix broken link

We already had the manual in our website static files, so not sure why
we weren't already linking to our own hosted copy.

tutorials/upperair_soundings.py

@@ -10,7 +10,7 @@
 problems. In this tutorial we will gather weather balloon data, plot it,
 perform a series of thermodynamic calculations, and summarize the results.
 To learn more about the Skew-T diagram and its use in weather analysis and
-forecasting, checkout `this <http://www.pmarshwx.com/research/manuals/AF_skewt_manual.pdf>`_
+forecasting, checkout `this <https://unidata.github.io/MetPy/latest/_static/USAF_SkewT_manual.pdf>`_
 air weather service guide.
 """