PEP 667: Format the flat_map example consistently (#2230)

pep-0677.rst

@@ -107,7 +107,10 @@ type checkers to find the bug.
 
 With our proposal, the example looks like this::
 
-    def flat_map(l: list[int], func: (int) -> list[int]) -> list[int]:
+    def flat_map(
+        l: list[int],
+        func: (int) -> list[int],
+    ) -> list[int]:
         out = []
         for element in l:
             out.extend(f(element))