fix(datepicker): add 'multi-year' to union type for startView property

[Nevaan]

Adds missing literal to union type. The issue pointed out that the possible value 'multi-year' is not listed on API page.

Fixes #11700

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[Nevaan]

CLA done

[googlebot]

CLAs look good, thanks!

[Nevaan]

Seems that travis builds fails, but not because of the code change ( response 502 Bad Gateway in one of e2e tests). Is this possible to trigger build manually?

also, mentioning @mmalerba as he asked on issue page

[Nevaan]

Travis-required fails also, but on the test of component I did not changed - how should I deal with it?

[crisbeto]

LGTM

[crisbeto]

Commit message fixup: should be marked as a fix instead of docs.

[Nevaan]

Message changed, thank you!

[julianobrasil]

Isn't this already included in #11049?

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