Document in-timezone coerces

PoV: rakudo/rakudo@c8438e6
Propspec: Raku/roast@529e4e0
Closes rakudo/rakudo#2381 R#2381

Author: zoffixznet

doc/Type/DateTime.pod6

@@ -375,7 +375,7 @@ Returns a DateTime object for the same time, but in time zone UTC.
 
 Defined as:
 
-    method in-timezone(DateTime:D: $timezone = 0 --> DateTime:D)
+    method in-timezone(DateTime:D: Int(Cool) $timezone = 0 --> DateTime:D)
 
 Returns a DateTime object for the same time, but in the specified C<$timezone>, which is
 the offset B<in seconds> from L<GMT|https://en.wikipedia.org/wiki/Greenwich_Mean_Time>.