Fix `trim_trailing_whitespace` typo in `.editorconfig` file (#6349)

trim_trailing_spaces -> trim_trailing_whitespace

trim_trailing_spaces seems to have been an older proposal for the name
of this property but trim_trailing_whitespace is what was finally
settled on. trim_trailing_whitespace already appears twice in this file.

https://github.com/editorconfig/editorconfig/wiki/EditorConfig-Properties#trim_trailing_whitespace

.editorconfig

@@ -26,4 +26,4 @@ end_of_line = unset
 indent_size = unset
 indent_style = unset
 insert_final_newline = unset
-trim_trailing_spaces = unset
+trim_trailing_whitespace = unset