(docs) fix error in formatting.md

[jacob-underwood]

code:

if (settings.text_case) {
    formattedString = change_case(formattedString, settings.text_case);
}

so technically an alternative could be to change the code to

if (settings.change_case) {
    formattedString = change_case(formattedString, settings.change_case);
}

but the change in the docs is probably easiest.