Fix key in Finnish locale

[wkmor1]

Key "Download plot as a png" was missing " a"
and therefore was not translated in Finnish locale.
@archmoj

Thanks for your interest in plotly.js!

Translations:

• Please @ mention a few other speakers of this language who can help review your translations.
• If you've omitted any keys from dist/translation_keys.txt - which means they will fall back on the US English text - just make a short comment about why in the PR description: the English text works fine in your language, or you would like someone else to help translating those, or whatever the reason.
• You should only update files in lib/locales/, not those in dist/

Features, Bug fixes, and others:

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[alexcjohnson]

Great catch @wkmor1 - this will close #4612. Also see the general issue about this #2269 if anyone is interested in ensuring we catch ALL issues of this nature.