doc/user: fix sphinx warning
WARNING: extlinks: Sphinx-6.0 will require a caption string to contain exactly one '%s' and all other '%' need to be escaped as '%%'.
Well, let's do that then!
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WARNING: extlinks: Sphinx-6.0 will require a caption string to contain exactly one '%s' and all other '%' need to be escaped as '%%'.
Well, let's do that then!