doc: fix typo 'BankAccount'

[zhongsp]

Fix a typo in TypeScript Language Specification word docs. Regenerate the spec.md file.

[msftclas]

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[mhegazy]

thanks @zhongsp. this file is generated from the word doc. can you update that one and run jake generate-spec

[zhongsp]

@mhegazy Thanks for your review. I've updated the code as your introduction.

[msftclas]

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[DanielRosenwasser]

@mhegazy did we confirm that running the markdown converter yields the same document?

[mhegazy]

yup.