Fix mistake about the reason of the limitation of the timestamp in 32bit systems

Author: Jérôme Deuchnord

reference/forms/types/options/_date_limitation.rst.inc

@@ -1,7 +1,7 @@
 .. caution::
 
     If ``timestamp`` is used, ``DateType`` is limited to dates between
-    Fri, 13 Dec 1901 20:45:54 GMT and Tue, 19 Jan 2038 03:14:07 GMT on 32bit
-    systems. This is due to a `limitation in PHP itself`_.
+    Fri, 13 Dec 1901 20:45:54 UTC and Tue, 19 Jan 2038 03:14:07 UTC on 32bit
+    systems. This is due to an integer overflow bug in 32bit systems known as the `Year 2038 problem`_.
 
-.. _limitation in PHP itself: https://www.php.net/manual/en/function.date.php#refsect1-function.date-changelog
+.. _Year 2038 problem: https://en.wikipedia.org/wiki/Year_2038_problem