update ssh-key section in user_accounts.rst
Contributor checklist
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My contribution is ready to be merged as is -
I verified the modifications render properly -
I used spell checking -
I used the styling and structure aids available in Sphinx/ResT. (Links use `…`_, enumerations#., lists*, code``…``/.. code::, warnings ..warning::, etc.) -
I reread the text keeping in mind that the text has to be comprehended fully by the target audience