[mongodb/en] Update mongodb.html.markdown

[arnotixe]

fixed mixed up $gte: and age under "Comparison operators" (and one count of duplicated $lte: -> $lt:)

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[arnotixe]

Sorry I didn't catch the $in / $nin in the same PR… not intentionally upping my PR count 😁

[vendethiel]

Your PRs can have several commits (or you can amend a previous commit to fold both into one).