Fix MAG entry in GEMPAK Guide

Currently, the row in the GEMPAK Conversion Guide referencing the MAG
function points to numpy.linalg.norm, but the actual NumPy equivalent
is numpy.hypot.  This commit updates the documentation accordingly.

docs/userguide/gempak.rst

@@ -544,7 +544,7 @@ blue is uncertain of parity, and white is unevaluated.
       <tr>
         <td class="tg-implemented">MAG(V)</td>
         <td class="tg-implemented">Magnitude of a vector</td>
-        <td class="tg-implemented"><a href="https://docs.scipy.org/doc/numpy/reference/generated/numpy.linalg.norm.html#numpy.linalg.norm">numpy.linalg.norm</a></td>
+        <td class="tg-implemented"><a href="https://numpy.org/doc/stable/reference/generated/numpy.hypot.html">numpy.hypot</a></td>
         <td class="tg-yes">Yes</td>
         <td class="tg-yes">Yes</td>
         <td class="tg-yes">Yes</td>