update links

Author: jmckenna

en/mapscript/php/migration_guide.txt

@@ -36,7 +36,7 @@ recommendation: as we saw with the breaking PHP 7 changes,
 managing PHP through SWIG will be easier on the MapServer maintainers in 
 the long run).  Some important SWIG notes:
 
-- you will require at least `SWIG-3.0.11 <http://www.swig.org/download.html>`_  
+- you will require at least `SWIG-3.0.11 <https://www.swig.org/download.html>`_  
   (but 4.0.0 is recommended).
 
 - if you notice any missing functions that were available in the old native PHP MapScript API,