[dart-dio][timemachine] Bugfix: Ensure closing bracket when adding serializers

[epou]

Using dart-dio with timemachine leads to syntax error in lib/src/serializers.dart -> Expected to find ')'

It seems the bug was introduced here #9180 (see file), where there's a closing bracket for {{#useDateLibCore}} ....... {{/useDateLibCore}} but not for {{#useDateLibTimeMachine}} ....... {{/useDateLibTimeMachine}}.

Adding the bracket just before the .build to ensure it is always closed, independent of what's defined in previous lines...

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[kuhnroyal]

LGTM, thanks!

[kuhnroyal]

@wing328

[wing328]

ci failure (already fixed in master) not related to this change