gui(web_services): Fix not all arguments converted during string formatting

[echoix]

Fixes a string causing problems after #4052, reported in #4052 (comment)

It was a simple typo, on my keyboard layout, $% are right next to each other.

[echoix]

I also verified, there are no other instances of "$s" that are the same kind of typo.
Even by enabling all ruff rules, and clearing the disabled rules of pylint, none of them would have caught that. Maybe because it is the result of calling a function, so it is difficult to be sure that the returned string would have placeholders.

If there were to have a pylint rule, it would be in a W130X or W13XX rule, like this one:
W1310: https://pylint.readthedocs.io/en/latest/user_guide/messages/warning/format-string-without-interpolation.html
W1301: https://pylint.readthedocs.io/en/latest/user_guide/messages/warning/unused-format-string-key.html

Removing the _() around the string makes pylint find the W1310, so it was the function call that hid it.