Firestore API: Fix return type name (unbalanced brackets) in pydoc.

[weisi]

Currently the return type is specified as Sequence[~.firestore_v1beta1.collection.CollectionReference[. Changing to Sequence[~.firestore_v1beta1.collection.CollectionReference]. (Note the difference in the last symbol.)

[googlebot]

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[weisi]

(Just signed CLA from the E-mail address that made the commit.)

[googlebot]

CLAs look good, thanks!

[tseaver]

@multiple1902 Thanks for the patch!