Document .earlier takes plural units too

.earlier's docs say to see .later for units and it's confusing
to say that only .later takes the plurals.

Author: zoffixznet

doc/Type/DateTime.pod6

@@ -274,7 +274,7 @@ converted to an L<Int>.
 Allowed units are C<second>, C<seconds>, C<minute>, C<minutes>, C<hour>,
 C<hours>, C<day>, C<days>, C<week>, C<weeks>, C<month>, C<months>, C<year>,
 C<years>. Please note that the plural forms can only be used with
-the C<later> method.
+the C<later> and C<earlier> methods.
 
 The C<:2nd> form of colonpairs can be used as a compact and self-documenting
 way of specifying the delta: