Fix parenthesis for optional function as return type (#11004)

Co-authored-by: fisker Cheung <lionkay@gmail.com>

changelog_unreleased/flow/11004.md

@@ -0,0 +1,19 @@
+#### Fix parenthesis for optional function as return type (#11004 by @vjeux, @fisker)
+
+<!-- prettier-ignore -->
+```jsx
+// Input
+const fun = (a: number): (?(string => string)) => {
+  return a > 0 ? s => `${s}: ${a}` : null;
+};
+
+// Prettier stable
+const fun = (a: number): ?(string) => string => {
+  return a > 0 ? (s) => `${s}: ${a}` : null;
+};
+
+// Prettier main
+const fun = (a: number): (?(string) => string) => {
+  return a > 0 ? (s) => `${s}: ${a}` : null;
+};
+```