fix(with-mongodb): Change comment of MongoDB client in getServerSideProps

[SokichiFujita]

Previously const client had been defined around the comment. Developer could easily see and understand what is the client. When try - catch was added, client also removed because of avoiding unused constant.

So currently developer may not be able to naturally understand what is the client.

Then I fix the comment to make the client clear.

Bug

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Feature

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Documentation / Examples

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[ijjk]

Thanks for the PR!