Update the link for cpt-city (#4389)

Author: seisman

pygmt/src/grd2cpt.py

@@ -66,7 +66,7 @@ def grd2cpt(
     *z*-value, the foreground color (F) assigned to values higher than the
     highest *z*-value, and the NaN color (N) painted wherever values are
     undefined. For color tables beyond the standard GMT offerings, visit
-    `cpt-city <http://www.seaviewsensing.com/pub/cpt-city/>`_ and
+    `cpt-city <https://phillips.shef.ac.uk/pub/cpt-city/>`_ and
     `Scientific Colour-Maps <https://www.fabiocrameri.ch/colourmaps.php>`_.
 
     If the master CPT includes B, F, and N entries, these will be copied into

pygmt/src/makecpt.py

@@ -49,7 +49,7 @@ def makecpt(
     CPT based on an existing master (dynamic) CPT. The resulting CPT can be
     reversed relative to the master cpt, and can be made continuous or
     discrete. For color tables beyond the standard GMT offerings, visit
-    `cpt-city <http://www.seaviewsensing.com/pub/cpt-city/>`_ and
+    `cpt-city <https://phillips.shef.ac.uk/pub/cpt-city/>`_ and
     `Scientific Colour-Maps <https://www.fabiocrameri.ch/colourmaps.php>`_.
 
     The CPT includes three additional colors beyond the range of z-values.