refactor: correct wrong import in date selection

[crisbeto]

Fixes an incorrect import in the date-selection.ts that breaks the build.

Fixes #13123.

@jelbourn I'm not too sure whether this is supposed to target the 6.4.x branch.

[jelbourn]

LGTM

[ngbot]

I see that you just added the pr: merge ready label, but the following checks are still failing:
     status "continuous-integration/travis-ci/pr" is failing

If you want your PR to be merged, it has to pass all the CI checks.

If you can't get the PR to a green state due to flakes or broken master, please try rebasing to master and/or restarting the CI job. If that fails and you believe that the issue is not due to your change, please contact the caretaker and ask for help.

[crisbeto]

Caretaker note: for some reason the CI is running against the lint rules from master.

[jelbourn]

LGTM

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