Update link to flow (#21862)

`main` branch does not exist in the `flow` project and it is `master`.

Co-authored-by: Ehsan Hosseini <[email protected]>

Author: e-hosseini

packages/react/index.js

@@ -7,7 +7,7 @@
  * @flow
  */
 
-// Keep in sync with https://github.com/facebook/flow/blob/main/lib/react.js
+// Keep in sync with https://github.com/facebook/flow/blob/master/lib/react.js
 export type StatelessFunctionalComponent<
   P,
 > = React$StatelessFunctionalComponent<P>;