Correct typo "intervall" to "interval" in lib_settings_strings.xml

There are, in fact, many more misspellings of "interval" within the code, but this is the only user-facing instance I can find. I don't know if it's worth refactoring the entire code over that misspelling if it won't appear to the user anyway. I may decide to do that at a later point, but this is a "quick win".