Improve/clarify backup code description text

[ssddanbrown]

Reported by Tim via Discord:

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The source string for mfa_option_backup_codes_desc is a bit broken
Securely store a set of one-time-use backup codes which you can enter to verify your identity.
This sentence indicates that a user needs to store the backup codes so they can use them later
Issue is the 'Securely' (and I guess 'store' as well)
In Dutch, you can't properly indicate an action that will be taken, and a suggestion for how to do so
e.g. the action is storing the codes, and the suggestion is to do so securely
that can't fit into one sentence like that
The current translation is just incorrect grammar
I would suggest changing this source string to something like
Get a set of one-time-use backup codes.... . Make sure to store these in a secure place.