docs: fix typo on MatBottomSheet page

[anseljh]

No description provided.

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[anseljh]

Signed!

[crisbeto]

LGTM, waiting for the CLA to pass.

[jelbourn]

LGTM

[jelbourn]

@anseljh I believe the bot needs the full message I signed it! to register

[anseljh]

I signed it!

[jelbourn]

Closing this since the CLA bot never got happy for some reason; I sent #20636 with the typo fix

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