Use use_parentheses in isort config suggestion

The `combine_as_imports=True` modifies isort style as a side-effect and was not the intended purpose of the suggested change in psf#250. The problem was that isort was actually replacing the parens with backslash and using `combine_as_imports=True` happened to also produce the same result.

The actual setting should be `use_parentheses` as this tells isort to use parenthesis for line continuation instead of \ for lines over the allotted line length limit and matches precisely what black is outputting.

README.md

@@ -237,7 +237,7 @@ the following configuration.
 multi_line_output=3
 include_trailing_comma=True
 force_grid_wrap=0
-combine_as_imports=True
+use_parentheses=True
 line_length=88
 ```